Under certain conditions, macOS may copy data into the void, leading to data loss of important files, all due to a bug in how the operating system handles APFS sparse disk images.
The issue was discovered and reported to Apple this past week by Mike Bombich, the creator of a Mac backup application named Carbon Copy Cloner.
Bombich says the bug affects only "sparse disk images" formatted in Apple's new filesystem format APFS, first deployed macOS High Sierra in March 2017. A "disk image" is a file format that contains the entire contents and structure of a disk volume, such as USB, CD, DVD, hard disk drive, or network share. The issue is that APFS sparse disk images fail to synchronize the value of the "free disk space" from the sparse disk image with the real "free disk space" value from the underlying hard drive.