iATKOS | United Kingdom: Serious Flaw in macOS May Cause Data Loss on APFS-Formatted Disk Images [Video]

Mike Bombich, the developer of the popular Carbon Copy Cloner application, has discovered a serious flaw in macOS that may cause data loss on APFS-formatted disk images.

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. Taking a closer look, I discovered two bugs in macOS's "diskimages-helper" service that lead to this result.

As demonstrated in the video below, Bombich found that an APFS volume's free space doesn't reflect a smaller amount of free space on the underlying disk and that the diskimages-helper application doesn't report errors when write requests fail to grow the disk image.