Mac – Time Machine local backups/snapshots stored

I haven't found any documentation for this feature yet, so I don't have any specific answers for your questions about interactions with external Time Machine drives.

However, I did do some playing around with the feature while using my laptop on battery. The first thing I noticed is that local snapshots were not enabled by default – for me, at least. I know that because "entering" Time Machine while my backup drive was detached resulted in the usual error message — until I executed the following from a command line:

sudo tmutil enablelocal

After I did this, a hidden volume called MobileBackups was created (in /Volumes) and I was able to "enter" Time Machine. (Perhaps local snapshots weren't enabled on my machine because I have less than 10% free disk space?)

Unfortunately, the feature seems a little half-baked. I tried the following, with pretty unsatisfactory results:

1. Created a new folder, ~/Documents/Test.
2. Forced a new snapshot by running tmutil snapshot.
3. Verified that a new snapshot was created by looking in /Volumes/MobileBackups/Backups.backupdb/hostname/Latests/drivename/Users/myname/Documents.
4. Deleted the Test folder.
5. Entered Time Machine and browsed the backups/snapshots. The Test folder was not there in any of the backups.

Update: I installed Lion on a second Mac, and without any explicit action on my part, this Mac has a MobileBackups volume. And restoring deleted files from a local snapshot works on this Mac.

I have seen this on several occasions and I have done this to clear up the corruption or glitch that prevents browsing local snapshots without an actual full time machine destination mounted:

1. Connect to one of my destinations and force a good backup

   tmutil startbackup --block --rotation
   

2. Disable local snapshots

   sudo tmutil disablelocal
   

3. Enable local snapshots - being sure the cleanup is done

   sudo tmutil enablelocal
   

I do watch the console app (or tail -f /var/log/system.log) during the backup and commands above to make sure that the local filesystem gets cleaned up and give the computer several minutes between the disable and the re-enable since it does have to unmount the network filesystem, delete old copies of files and clean up things before trying again with a local snapshot.

The downside of this is you do lose any copies of files that only existed on the local snapshot intervals and didn't get saved to an actual remote location. It also doesn't really dig into why the store became corrupt so you might have the same thing happen again at a later date. I've also had good luck with reproducible time machine problems reporting it to Apple for assistance. They'll want you to run tmdiagnose and probably not reset things like I mentioned above if you want to get at the root cause of what really happened as opposed to getting it working again.