I have not had a freeze like that for a couple of weeks - Applecare wanted to pin it on a bad USB hub, but I have run with and without the hub between the MBA and the Time Machine external drive with no difference in behavior.
I currently have an issue with not being able to eject the Time Machine drive unless I force it to eject. The symptoms are as follows from a few days ago:
A Reboot allows the external USB drive and all of its volumes to be ejected and I can remount them with Disk Utility
If I turn Time Machine on, enter Time Machine, cancel Time Machine and eject the drive I have no problem
Remount Time Machine drive with Disk Utility
Perform "Backup Now" with Time Machine then stop it from the Finder and eject the drive - works fine.
Remount Time Machine drive with Disk Utility
Perform "Backup Now" with Time Machine then stop it from the Time Machine drop down menu and eject the drive - works fine.
Remount Time Machine drive with Disk Utility
Perform "Backup Now" with Time Machine and let it run to completion then try to eject the drive - get the following Console messages:
7/8/12 8:14:02.985 AM com.apple.backupd: Backup completed successfully.
{after this message I open Finder and try to eject the Time Machine drive; get a box asking if I want to dismount both partitions; answer yes; get box saying that it can't unmount Time Machine because something may be using it followed by the next two messages}
7/8/12 8:14:49.156 AM com.apple.backupd: Stopping backup to allow backup destination disk to be unmounted or ejected.
7/8/12 8:14:50.059 AM com.apple.backupd: Backup failed because the destination disk was ejected or disconnected unexpectedly!
These two messages appear to be in conflict: if "Backup completed successfully" then why is the next line announcing that my attempt to eject the drive is causing the backup process to be stopped?
Perhaps there are two different processes involved in the backup and one reports completion while the other one never completes and continues to tie up the drive?
Is anyone else seeing this kind of behavior?
Oh yes - the crashes that I am getting from time to time seems to associated with the process "mds" the indexing routine that works for both Time Machine and Spotlight.