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:
- Created a new folder,
- Forced a new snapshot by running
- Verified that a new snapshot was created by looking in
- Deleted the
- 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.