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Time Machine BUG: Silent Exclusion of Volumes

Last updated 2012-12-04 - Send Feedback
Related: Apple macOS, backup, Time Machine

This bug is extant as of early December 2012 in OS X 10.8.2 Mountain Lion (and almost certainly on Lion as well). A bug has been filed at Apple.

Apple’s Time Machine is a valuable feature which I recommend, but definitely not as the only backup. See the articles on Time Machine.

In early spring to summer 2012, I began noticing a Time Machine bug which could lead to data loss by virtue of no backup. This occurred with OS X Lion and still occurs as of August 2012 with OS X Mountain Lion.

On my Mac Pro, I use one volume Boot for system and applications, and another volume Master for my current and critical files. Both volumes are internal to the Mac Pro. I also have a number of other volumes, but I exclude these from Time Machine backup, because I back them up via cloning, and because they don’t change much (TM is best used for protection of recent changes/additions only, not as a full backup system).

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The bug

Time Machine has repeatedly and silently added both my Boot and Master volumes to its “Exclude these items...” list.

This is not a one-time thing, I have seen it happen 15-20 times in 3 months. This includes after erasing the backup drive and the original volumes, so it is not tied to a particular volume; it is a general bug. With OS X Lion and Mountain Lion.

Excluded this way (silently and without warning), the hapless user (me) thinks that a regular hourly backup is happening when in fact it is not, the danger being that this could go on for days or weeks or months.

Most recently, Time Machine auto-excluded these critical volumes and then posted a dialog saying there was nothing to backup— this time I was lucky. Usually, Boot remains (so no such warning is issued), but Master alone is excluded— my most critical files left unprotected (in the short term, I do make clone backups regularly).

Even nastier, when I open the Time Machine preferences and un-exclude Master (after TM excluded it on its whim), it pops right back into the excluded list a short while later.

The day after I wrote this article, Time Machine screwed up yet again, placing my boot volume BootA and my critical data volume Master into its excluded list. This is rotten to the (Apple) core.

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Reader Terry W writes:

You can see this in action— mount a disk that you want to exclude, then add it to the exclusions list, save the change, then restart your computer.

Mount the disk that you have excluded, open the exclusions list again — the disk will be listed as expected. THEN unmount the disk. Magic: the excluded disk changes from the one you saved to one of your currently mounted disks!

DIGLLOYD: Unbelievable incompetence at Apple.

Mitigating the risk

Time Machine is one of those “it just works and we know what’s best for you” iNanny features, so there is no email notification of successful backup, no option to post an alert (say once a day) summarizing things, etc.

And the list of excluded items is quite long on my system— another abject design failure— no opt-in, only opt-out, which makes a big messy list in a tiny window, making it hard to spot errors like one more item suddenly appearing in a long list.

Since Time Machine does not work reliably for me, I mitigate the risk in two ways:

  • I make a clone backup of Boot and Master at least once a day, using Carbon Copy Cloner (scheduled).
  • I check the Time Machine “Exclude these items...” list regularly (a band-aid approach, all I can do is catch it so that not too long an interval passes).

Possible causes

I have not been able to nail down a causal factor. Some things come to mind:

  • I have up to ten internal volumes. Perhaps Time Machine simply has a bug in some kind of internal list (say an array indexing or looping bug).
  • Perhaps there is some side-effect of cloning a backup that confuses two volumes (they do all have different names). But TM identifies volumes by a unique ID, so this this does not make sense either.
  • Perhaps the nature of the volume has an influence (e.g. RAID-0 stripe). But Time Machine ought to be looking at volumes, not drives, so I deem this idea unlikely also.

Reader comments

Peter writes:

I've not seen the ™ problem you are having, but been experiencing one just as bad:

I find some newly added folders/ files NOT being backed up - usually my D700 image files (while my new point & shoot image files newly added ARE backed up). I use 'on demand' ™ backup, not continuous backups. to 'force' ™ to find these missed files, i have to either (i) create a dummy blank folder in the affected folder, then ™ will 'see' it, and back up the whole folder, of (ii) go into preferences, and re-setup ™. either way a pain - and what use is a backup that you have to double check afterwards? since my image files are critical, i always check to see if they were backed up, but who knows what else is NOT being backed up?!

I had this bug in SL, and still not fixed in Lion

If not bad enough, in another episode i had a MASTER (data) disk go bad, so i restored from my 1 day old ™ backup. it concluded restoring the files, no error messages, copying over 2TB of data to my new replacement blank drive. FORTUNATELY i did a compare between the time machine source, and the restored disk destination. turns out 3 folders of images were NOT restored. i then manually told ™ to restore these 3 folders, and it was restored w/out a problem (indicating the files were all fine). this is scary stuff! :-(

DIGLLOYD: Sounds scary to me also.

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Michael H writes:

Interesting article on Time Machine malfunctions.

I, too, have had numerous failures of Time Machine over the last year. I haven't rigorously determined or recorded causes but the experience is the same. I think life is good, that Time Machine is working away in the background until one day when I go to retrieve something and discover that a backup hasn't been created for eons.

Just this last week I had another occurrence of Time Machine collapse. In this case, Time Machine had removed one of my backup volumes from the exclude list (do I even need to mention that it is terrible for Time Machine to automatically-with-out-requesting-verification add an entire volume to the Time Machine back up?). As a result, my Time Machine volume was too small for the back-up, so what does Time Machine do? Wipes out the back-up history. Arg. My backup volumes are bare drives connected via a NewerTech eSATA drive dock.

DIGLLOYD: Lots of room for better control here, if Apple wills it.

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