Reader Geof points out that you can get some idea of what Time Machine is up to - when it is performing backups, and how large each backup is - simply by looking at the Console. Open the Console (in the Utilities folder) and click "All Messages". Now type "backupd" (the name of the background process that does Time Machine's backing up) into the search field, and you'll see all and only the recent log messages from Time Machine.
Here are some typical messages, from my own machine (the dates and times have been omitted, for clarity):
Backup requested by automatic scheduler Starting standard backup Backing up to: /Volumes/SecretSharer/Backups.backupdb No pre-backup thinning needed: 969.3 MB requested (including padding), 119.97 GB available CoreEndianFlipData: error -4940 returned for rsrc type FREF (id 133, length 7, native = no) Copied 1917 files (157.5 MB) from volume Hume. No pre-backup thinning needed: 795.7 MB requested (including padding), 119.81 GB available Copied 466 files (102 bytes) from volume Hume. Starting post-backup thinning Found partially deleted backup - trying again to delete: 2007-11-12-114754 Deleted backup /Volumes/SecretSharer/Backups.backupdb/hume/2007-11-12-114754: 119.78 GB now available Post-back up thinning complete: 1 expired backups removed Backup completed successfully.
This is really quite lovely. You learn how much material was copied, whether any expired backups were deleted, and how much room remains on the backup volume. There are a couple of rather worrying messages in this collection (the CoreEndianFlipData error does not not look normal, and why was there a "partially deleted backup"?), but on the whole the messages are simple, clear, informative, and rather comforting, in contrast to the rather secretive surface behavior of Time Machine, which always leaves a note of doubt, in my mind at least, as to what precisely is going on ("What's that disk noise? Is it Time Machine operating? What's it doing?").
Geof adds that the Time Machine Console messages on his machine resolve a question we posed earlier. We said:
Some people are finding Time Machine won't perform an automatic backup from a portable computer if the portable is running on battery power (though it will perform a manual backup when requested to via the Time Machine Dock menu). This could be deliberate; perhaps Time Machine would prefer not to initiate a time-consuming, sleep-preventing activity and risk draining the battery.
Geof's own Console messages confirm our conjecture (again, the dates and times have been removed):
Backup requested due to disk attach Not starting automatic backup, running on non-AC power source. Backup requested by user Starting standard backup
This tells you exactly what happened. Geof plugged in his external drive, and Time Machine took this as an invitation to perform a backup. But since he was running on battery power, it decided not to - thus proving that this is, indeed, a deliberate policy. He then manually requested a backup, using the Time Machine Dock menu; and Time Machine obeyed. So, as we guessed, when you're running on battery power, Time Machine won't do any automatic backups, but it will do a backup if you force it to, all of which seems a sensible policy.