Apple’s New APFS File System in macOS High Sierra Can Cause Loss of Data – But You’re Highly Likely Not Affected By it

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Feb 19

A bug found in Apple’s APFS file system in macOS High Sierra could cause loss of data. But it does not affect everyone. Only specific use-cases.

Apple’s APFS File System Bug Affects Sparse Disk Images – Startup Volumes Remain Absolutely Unaffected

If you are a macOS High Sierra user and make do with ‘sparse’ disk images then you’ll be surprised to learn that a bug causes the entire OS to inaccurately track free storage space, causing data to be lost for good if not tracked properly by the user.

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First discovered by the creator of Carbon Copy Cloner, Mike Bombich, the bug is very deceiving to users. For instance, you may write an entire file on a sparse disk image and it would appear as though everything has gone through absolutely fine even if the physical storage space is not there to write it.

In Mike’s case, he copied a video file, double-checked whether or not everything was copied just fine, and even played back the video and everything seemed OK at first. But the problem began to manifest itself when he unmounted the sparse disk image and remounted it, only to find out that the video he copied was totally corrupted.

Here’s how it went down in Mike’s own words:

Earlier this week I noticed that an APFS-formatted sparsebundle disk image volume showed ample free space, despite that the underlying disk was completely full. Curious, I copied a video file to the disk image volume to see what would happen. The whole file copied without error! I opened the file, verified that the video played back start to finish, checksummed the file – as far as I could tell, the file was intact and whole on the disk image. When I unmounted and remounted the disk image, however, the video was corrupted. If you’ve ever lost data, you know the kick-in-the-gut feeling that would have ensued. Thankfully, I was just running some tests and the file that disappeared was just test data.

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Interestingly, this issue has been reported to Apple by Mike. But when a fix for this problem will arrive is anyone’s guess at this point.

Should you be worried about this bug? Not at all, for now. What if you’re a Carbon Copy Cloner user? As it turns out, only 7% of CCC backups rely on sparse disk images while 12% of those fall under the APFS file system. So, you shouldn’t be bothered much by it at all. Last but not the least, is my main volume affected? Nope, startup volume remain absolutely unaffected by this bug.