This unusual error was recently discovered by Mike Bombich, the creator of Carbon Copy Cloner, which is a popular Mac backup application. According to Mr. Bombich, the bug is only likely to impact a small percentage of users, but if you’re one of the unlucky few who lose important data, that’s going to be small consolation.
Here’s how the bug works, and where it can get you into trouble:
Let’s say you’ve created a 100 GB APFS-formatted sparse image whose data is stored on a remote network share.
Time passes, and you copy 90 GB worth of data to the remote network share, which leaves just 10 GB for use by your sparse image, but therein lies the problem. The sparse image still thinks it’s got the full amount of space to play with.
At this point, if you copy a 20 GB file, the copy function will appear to succeed. In the short run, you’ll still be able to access and open the file until you reboot your machine. After restarting, 10 GB of the 20 GB file copy vanishes, and the file becomes corrupt and unusable.
Details of the bug have been forwarded to Apple, and the company is in the process of reviewing them. At this point, no ETA has been given on when a fix might be made available. Until it is, be very careful when using sparse images, because the system will let you copy your files right into oblivion.