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Understanding Apple's Time Machine

This tip is to help everyone understand how Apple's Time Machine application (it's actually more of a system service) works, what to expect from it, and what it's limitations are.

The first thing to understand is the difference in terminology. Apple produces a piece of hardware called the "Time Capsule". This piece of hardware is a combination of the Apple "AirPort Extreme" Base Station Wifi Router and an external Hard Drive. The application "Time Machine" is a piece of software that runs on every Mac OS X 10.5 & 10.6 system and is the software that automates the process of copying files from your internal Hard Drive to a backup location.

"Time Machine" the application can backup your files to many different possible places. Your flies can be backed up onto most USB or FireWire External Hard Drives, another Internal Hard Drive, a Network Attached Storage (NAS) Device, a Apple File Sharing (AFP) volume Shared by another Mac, or yes even an Apple "Time Capsule".

There are 2 Things you need in order to use "Time Machine"..
1. An Apple Macintosh running Mac OS X 10.5 Leopard or 10.6 Snow Leopard
2. A device that will store your backed up files. (See "What Device To Use.")

How Time Machine Works

The following slide show shows the basics of how Time Machine backs up files as you create them. In this scenario we have a file that is created at 11:30 AM. At 12:00 PM Time Machine performs it's first backup, and as the file is edited Time Machine continues to backup the file every hour. It is possible to have MANY copies of one file on your Time Machine HD. If the Time Machine HD begins to get full, Time Machine (the app) will begin to delete the older versions of backed up files. Time Machine will however ALWAYS keep the last version of a file that was backed up, even if the original file is deleted off your computer's internal HD. Again, Time Machine will begin deleting backups ONLY when the Time Machine HD begins to get full.

Time Machine File Deletion Schema

An important fact to remember is that a file is not "backed up" unless it is stored on two hard drives at the same time. If you delete the original file off your computer's internal HD it is no longer backed up. Yes, the file does still exist on the Time Machine HD, but if the Time Machine HD would fail in any manner there is no longer a copy on your Internal HD. In this way, your Time Machine HD is also a Storage HD, but don't rely on it as such.

With the above sentences in mind it is important to remember that since Time Machine will never delete the final copy of any file this is created on your Time Machine HD, your Time Machine HD may fill up very quickly unless steps are taken to reduce the number or size of files that are created, renamed or moved. For instance... if you create a file called "A.doc" on your desktop Time Machine will create a file called "A.doc" on your Time Machine HD. If you simply rename this file to "A1.doc" Time Machine will now permanently store a copy called "A.doc" and another file called "A1.doc".

This is not a problem when it comes to Word files or any text file in general as their sizes are small enough you likely would not notice the backup HD space being affected. Conversely, if you are working with an Adobe Photoshop file (.psd) that is 300 MB in size you Time Machine HD will likely fill up very quickly. Each hour you work on the file and alter it Time Machine will back up that hour's version of the 300 MB file. Thusly if it takes 4 hours to complete the file, Time Machine could have created well over 1 GB worth of backups. My suggestion is to use a folder called "In Progress" and another folder called "Archive". When you work on files save them only into the "In Progress" folder. Only once complete should the file be moved to the "Archive" folder. The reason this can solve the problem is because you can then simply delete the "In Progress" folder from your Time Machine HD. Doing so will delete all files and all copies of files from within the "In Progress" folder and then the next hour Time Machine will recreate the "In Progress" folder with it's new contents.


How Time Machine Stores Files On Your Time Machine HD
This column shows the content of the Macintosh HD. The original file that is created and the subsequent editing of that file over several hours. Time Machine runs every hour and backs up new files or files that have been edited / altered. This column shows a representation of the Time Machine HD contents when browsed by a user viewing the contents through the Finder. In the Finder a user will see a folder for every hour Time Machine has run. In each of these folders it looks like there is a complete copy of your Mac's HD. Time Machine uses "Hard Links" to make it appear as though files exist in multiple locates, when in fact there is actually only 1 copy of each version of any file. There could be 1000 Hard Links pointing to the same data file on the Time Machine HD. This column shows a representation of the actual data stored on the Time Machine HD's platters. As you can see even though there are many Hard Links that appear in the Time Machine HD Column, there are only 3 actual files being stored on the Time Machine HD File System.

What Device to Use

So what type of back up drive should you use? There are Pros and Cons to each type of back up device you could use, so let's break down the possibilities. The diagram below shows the ability of each device to support different features. Each device gets scored by Best, Good, Mmhhh.. (Mediocre) and Not So Hot on it's support of the features listed.

Time Machine Media Type Review

Understanding the features in the above diagram:

SPEED: Whether or not back ups to this device would be fast. Speed depends on many different variables. This rating is strictly based on throughput to the backup device(s) I tested.

CAPACITY: Does the type of device have sufficient space to backup your files or have the ability to be expanded if more space becomes necessary.

STABILITY: Is Time Machine going to be reliable while backing up to this type of device.

EASY SETUP: Can this type of backup solution be setup easily.

COMPATABILITY: Whether or not this backup solution has wide spread compatibility with the majority of users.

PORTABILITY: Can you take it with you. Some solutions become integrated into your network (ie Time Capsule) so much so that removing them would become very difficult. TIP: Never take your Time Machine backup with you when you travel. If you must take it with you carry it in a separate bag. The reason? If you bag containing you laptop is stolen with your computer and backup HD inside it your data is gone! Don't keep all your eggs in one basket. This feature ties into the next reviewed feature.

MEDIA REDUNDANCY: Can the Time Machine HD Media be swapped out easily. This is important as many businesses keep an onsite backup and an offsite backup of their data. This ensures that if one copy was stolen or destroyed they would have a second copy of the backup from which they can reload missing data. To achieve this "Media Redundancy" you simply need two Hard Drives with the SAME name. When one drive is removed and the other is plugged in Time Machine will fill in the missing files from the last time the current Time Machine HD was used.

WIRELESS: Can this backup device be accessed wirelessly?

WIRED: Does this device support physically be plugged-in. This is important because no matter what device you have the fastest method for backing up is via a hard line connection to your backup device.

AVAILABILITY: Can these devices be acquired easily, and can they be used in the majority of your systems. For instance, Internal HDs are widely available, but there are very few models of Mac that have the capability of having a 2nd Internal HD installed into them.

MULTIPLE SYSTEMS: Can this backup solution support Multiple Systems / Computers. This is important for many businesses as they may have 5 to 10 computers backing up to the same Backup Volume. This reduces overall cost as it saves the business from having to buy a HD for every machine in the office.

ENCRYPTION: Does this backup solution support Encrypting the Time Machine backup? Not a great idea... but important to some. Do remember that encrypting your Time Machine backup will slow the backup process down. Also, if the encryption key (aka Password) for the .sparseimage is lost or forgotten your files are just as gone as if the Hard Drive have failed. Do this at your own risk, I don't recommend it!


Types of Backup Solutions

1. External Hard Drives (USB or FireWire)


A. Very Fast
B. Very Large Capacities Available
C. Very Stable
D. Easy Setup (By far the easiest method to setup.)
E. Maximum Compatibility
F. Easily Detached and moved offsite for data protection assurances.
G. Easily rotate between multiple backup drives for onsite and offsite copy of data.
H. Inexpensive


A. Dedicated to one computer unless physically moved. (or used as AFP Host)
B. Requires more cables to always be connected to the computer.

2. Internal Hard Drive


A. Extremely Fast
B. Very Large Capacities Available
C. Very Stable
D. Easy Setup (Post HD Installation)
E. Inexpensive


A. Very few machines support additional Internal HDs. (Currently ONLY Mac Pros, Xserves)
B. Dedicated to one computer. (unless it is used as AFP Host)
C. Can NOT be easily taken offsite for data protection assurances.

3. Network Attached Storage (NAS) Device


A. Can Service Multiple Computers
B. Accessible Wirelessly (No Cords attached)
C. Easily Detached and moved offsite for data protection assurances.


A. Limited Speed (Based on Network Bandwidth / Connection & Device Specs)
B. Most NAS Devices don't support HFS+ Journaled Case Sensitive Format
C. Most use SMB Protocol, which limits file size and can cause issue with the Sparse Bundles Time Machine creates.


4. Apple File Sharing (AFP) volume Shared by another Mac

See "Time Machine Backup Over A Network" for more information.


A. Can Service Multiple Computers
B. Accessible Wirelessly (No Cords attached)
C. Allows for Encrypting the "Time Machine" backups (Nice feature, but I don't recommend it.)
D. Accessible over the Internet with a little setup. (Primarily via MobileMe but not necessarily.)
E. Inexpensive (as long as you already have another Mac to act a the host.)


A. NOT Easily Detached and moved offsite for data protection assurances.
B. Limited Speed (Based on Network Bandwidth / Connection)
C. Requires at least 2 Macs to set up. (Host and Client)
D. Host Mac must be powered on in order for Time Machine to back up your files.

5. Apple "Time Capsule"

See "Time Machine Backup Over A Network" for more information.


A. Can Service Multiple Computers.
B. Accessible Wirelessly (No Cords attached)
C. One piece of Hardware acts as your Router and your Backup Device.
D. Accessible over the Internet with very little setup. (Access ONLY, not actually backing up.)
E. MobileMe integration


A. NOT Easily Detached and moved offsite for data protection assurances.
B. Limited Speed (Based on Network Connection & Device Specs)
C. One piece of Hardware acts as your Router and your Backup Device. (one goes it all goes)
D. Limited capacities (2 TB or Less)
E. Expensive Price (On a Per GB Level at least)


Time Machine Backup Over A Network

The following diagrams show several scenarios that can be used to back up your computer via Time Machine over your work or home network. There are many advantages to backing up via your network, however it may not be your best solution. Take a look and see if a network backup may work for you. Regardless of whether you end up using a Time Capsule, AFP Volume from another Mac, or another NAS product, I highly recommend doing the first initial backup via a wired network connection. Wireless networks, although fast these days, are still no where near the speed of Ethernet Networks.

In order to back up to any volume, Time Machine needs that volume to be mounted locally. When you select a Network Volume as your Time Machine's backup destination, Time Machine will create a .sparsebundle file on that Network Volume. Time Machine will then mount the newly created Sparse Bundle Image into your computer local file system. This allows Time Machine to have all the write permissions it requires in order to backup your files. This is done for technical reason and is a method that is used to overcome some of the limitation of many of the Network Attached Storage (NAS) products. Many NAS products offer SMB services and use a FAT32 based File System for their local storage. This introduces many problems such as file name length, character support, file size limitations and so on. That is where the Sparse Bundle comes in. The "YourComputerName.sparsebundle" is actually a folder containing thousands of files that are 8.4 MB in size or smaller. This Sparse Bundle is a form of Disk Image. The same technology that is widely used to distribute Mac Software on the internet.


Time Machine Backup Network Scenario 1 Wired Gigabit Ethernet

The above Network Scenario is by far the fastest network based method. In this scenario I used my MacBook Pro as the Client, and a Mac mini Core Duo as the host. The Mac mini had a FireWire HD attached to it that was used as the final destination for my Time Machine backups. In this test Time Machine achieved a maximum throughput of 26.08 MB per second. The network was able to sustain 14 to 15 MB per second.



Time Machine Backup Network Scenario 2 802.11n 5 Ghz Wireless



Time Machine Backup Network Scenario 3 802.11n 5 Ghz Wireless Encrypted



Time Machine Backup Time Capsule Scenario 1 802.11n 5 Ghz Wireless




Time Machine Backup Time Capsule Scenario 2 Gigabit Ethernet></p>
<h3><strong>In conclusion</strong></h3>
<p>Whether you use a USB Hard Drive, a NAS, or a Time Capsule. Please back up your files!</p>
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iCanHelp! provides Apple Macintosh technical support and Mac Repair in the Brainerd Lakes and surrounding areas in Central Minnesota. (MN). iCanHelp!'s service area includes but is not limited to Brainerd MN 56401, Baxter MN 56425, Pequot Lake MN 56472, Nisswa MN 56468, Pine River MN 56456.

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