One of the new features included with Lion is the addition of “local snapshots” to Time Machine. Every hour Time Machine creates backups directly on the system’s hard disk. It doesn’t matter if an external Time Machine drive is connected.
The reasoning behind this feature seems to be that users may need to restore files at times when they don’t have access to the drive used for Time Machine. I could write more lines about why this feature might be useful but ultimately I don’t think this will be a heavily used feature for most. It’s also difficult to understand why the option to enable or disable local snapshots was removed from the Time Machine preferences pane prior to the final version of Lion.
Many owners have pointed out that the hourly backup can waste processing power and hard drive space. Overall, I just don’t see it being very useful for the majority of users. How often do you delete a file and then realize hours later that you shouldn’t have? I’m not stating that it never happens, but I don’t think it happens often enough to justify the addition of this feature (nor to remove the ability to change it from the GUI).
According to Apple’s support documentation the local snapshots are automatically enabled on portable Macs but are not enabled for desktop Macs. After inspecting my computers this does seem to be the case. Time Machine on the iMac doesn’t show any gray colored Time Machine backups. On the MacBook Pro I was able to enter Time Machine without the external drive connected. However, after I disabled the local snapshots option I can no longer enter Time Machine without an external drive connected.
As I mentioned, the ability to disable local snapshots isn’t available in the Time Machine system preference. However, it is possible to turn it off from a shell prompt, which can be accessed using Terminal.
Disable Time Machine Local Snapshots
sudo tmutil disablelocal
In the list below I’m also including a link to another blog in which the author describes his reasons for disabling local snapshots.