A bug in Time Machine is causing Apple's backup solution to exclude certain volumes from the backup. This could lead to data loss, warns a report.

According to the report on MacPerformanceGuide, Time Machine can sometimes miss critical volumes from the back up without warning.

Lloyd Chambers wrote: "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."

He explained that even after un-excluding a volume from the list "it pops right back into the excluded list a short while later."

Chambers claimed that he has been experiencing the issue in Mountain Lion since the beginning of December, and in Lion since the spring. He said he has bought it to Apple's attention.

He admited that he has a number of volumes that he excludes from Time Machine backup, because he backs them up separately. Normal users are unlikely to have multiple volumes for backup.

Chambers suggested that a possible cause of the bug could include the fact that he has around ten internal volumes. "Perhaps Time Machine simply has a bug in some kind of internal list." Another suggestion: "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."

He also questioned whether the issue is the nature of the volume – whether it has a RAID-0 stripe. But, he concluded: "Time Machine ought to be looking at volumes, not drives, so I deem this idea unlikely also." 

Apple's Time Machine is a backup utility that is built into OS X. It creates hourly backups of files, recording changes, rather than creating a clone. With Time Machine users can revert to the way their system was at any point in the past, either recovering a lost file or undoing a recent change – particularly useful in the case of a flawed update or an failed attempt to rebuild an Outlook database. Time Machine is also useful for migration of an old Mac to a new Mac.

One of the settings of Time Machine allows users to exclude certain file or folders from the back up, to save space for example. To add a file to the exclusion list go to Time Machine in System Preferences > Options > + > [select folder/file names].

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