Mac – Time Machine: Local Snapshots and Backup while on Battery

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.

This site is devoted to answering questions about Apple Products, but it's aimed more towards comsumers than power users. That doesen't mean your question is "wrong" in any way or you should remove it, but I strongly reccomend to ask it at the Super User Stackexchange site, which is devoted to power users like you. I think you might find there someone who can answer your question much sooner than here.