Time Machine uses a unique ID (UUID), which is specific to a volume (i.e. a partition of a disk), to associate the volume with its backup. In OS X releases before 10.7 Lion, when people exchanged their disk or migrated to a different Mac, this feature has made it difficult to get Time Machine to continue adding to the backup history of the previously used disk.

In OS X Lion and now also OS X 10.8 Mountain Lion, there is a nifty new Terminal command called tmutil, which makes this whole drama a breeze.

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The following procedure allows you to create a folder /Users/Shared/Media (referred to as the media folder) with special access control settings that have the following effect: *Any file or folder that is copied into the media folder becomes available to any current and future users on the same Mac. This means that all users will be able to edit, move, rename, and delete the files and folders within the media folder.*

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Apple’s Mail.app still does not support multiple sender identities, such as for example “John Doe john.doe@example.com” and “Johnnie johnnie@example.net” through the graphical user interface. However, there is a hint how it is done in Apple Mail up to and including Snow Leopard.

In Mac OS X Lion, Mail has reached version 5 and Apple has introduced a new layout for Mail’s Library folder, which is called “V2”.

Here’s how you can still use multiple identities in Lion.

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Lion lists recent items with each application in the Dock (accessible via right-click) and in the Application View of Mission Control. To remove those and disable future additions, the trusty defaults write approach may help with most — but not all — applications.

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Mac OS X 10.7 Lion introduced File Fault 2, a whole-partition encryption scheme. Depending on your disk and CPU, encryption may causes a significant performance hit.

Here’s how to compare the speed of disk access once you have enabled encryption with the speed without encryption.

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Safari offers a “Private Browsing” mode, in which it does not remember any history or cookies. While this can be very useful for online banking or other activities that should not leave behind any traces, enabling it is a pain: there is no keyboard shortcut, and Safari asks you before it enables the Private Browsing mode.

Fear not: Mac OS X, beginning with Snow Leopard, allows to define your own keyboard shortcut and you can define one that skips the confirmation dialog.

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Snow Leopard brought built-in support for Cisco VPN over TCP (but not over UDP). However, as of 10.6.6, there is still one issue: While the password can be saved in the keychain, the daemon configd is not granted access, causing the user to be bothered to enter the password every time upon initiating a connection

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The built-in Cisco VPN client introduced in Mac OS X 10.6 Snow Leopard has the habit of disconnecting itself when re-keying should be performed but fails after about 48 minutes up to one hour. This timeout issue has been fixed in Mac OS X 10.6.4.

In versions of Mac OS X 10.6.0 up to 10.6.3, the problem can be worked around by increasing the re-keying period to e.g. 1 week.

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If you clone a volume that you backup with Time Machine, its UUID will change and Time Machine will no longer be able to make backups of it. However, it won’t tell you directly what the problem is. Rather, you will find entries like the following in /var/log/system.log:

	backupd Backup failed with error: 12
	backupd Error (12): Link of previous volume failed for macosx.

If you want to continue to backup to the same volume in your Time Machine backup database, the following script allows you to change the UUID of the existing backups, leading Time Machine to backup as if the volume’s UUID had never changed.

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QuickLook is the technology behind the handy window that pops up when you hit the space bar in Finder or many other applications. Apple includes preview generators for the most popular file formats; however, there are also a few very useful so-called QuickLook Generator plugins from third parties.

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