After Mountain Lion upgrade: time machine cleaning up... for how long?

It may be having trouble indexing your backups, or deleting "expired" backups.

A clue may be lurking in your logs. Use the widget in #A1 of Time Machine - Troubleshooting to display the backup messages from your logs. Locate the backup in question, then copy and post all the messages here.

iMead wrote:

. . .

Error: Error Domain=NSOSStatusErrorDomain Code=-50 "The operation couldn’t be completed. (OSStatus error -50.)" (paramErr: error in user parameter list) deleting backup: /Volumes/Time Machine Backups/Backups.backupdb/Eric’s MacBook Pro/2012-07-28-155140.inProgress/DEAC22F7-CD3F-4F48-9A39-9667396B5986

Ah, it's an ".inProgress" package. That's the leftover from one or more failed or cancelled backups. When a backup does complete normally, TM will delete the partials. It's rare for that to fail, but that's what's happened.

You should be able to delete that manually, but (especially since your backups were made over a network), it's a bit of a pain:

1. Connect the drive directly to your Mac.
2. Open it via the Finder. There's a sparse bundle named for your Mac. Double-click it to mount it.
3. A Time Machine Backups disk image should appear on your desktop and/or Finder sidebar.
4. That may take a few moments. Double-click that to open it.
5. Inside is a Backups.backupdb folder, containing one folder named for your Mac.
6. Inside that is a series of date-stamped backup folders, plus a Latest alias, plus the ".inProgress" package we're looking for.
7. Delete it.
8. Hold the Alt/Option key while emptying the trash. That will take a while.

By the way, I hate to have to tell you this, but backing-up that way is unreliable and not supported by Apple. See: Using Time Machine with an Airport Extreme Air Disk.

iMead wrote:

. . .

Ha! Really? I was unaware that it was unsupported. Go figure. Guess they want you to get a Time Capsule? Or just not use a networked drive as a backup. Love Apple products but that just puts a bad taste in my mouth. Oh well.

As noted in the link, it does work for some folks, for some period of time. But (apparently due to different internal hardware vs. a Time Capsule) for most, sooner or later the backups turn up corrupted.

As an experiment, I backed-up that way for about 3 years (in addition to lots of redundant backups). Even with a good connection, near the Airport, no other networks in the area, and both Mac and Airport on U.P.S. systems, the backups got corrupted every few months. It ranged from about six weeks to about 7 months. That's a very common scenario.

It's always a bit risky to depend on a single set of backups, but even more so in that setup. See Time Machine - Frequently Asked Question #27 for some suggestions.