macOS
Shining a spotlight on the solutions to your most irritating Mac problems
Which disks get converted by Catalina?
Q Would my iMac with an internal Fusion Drive upgrade fully to Catalina, and what would happen with the Time Machine backups on its external hard disk: would those be converted to Apple’s new file system?
by CLIFFORD MOISEY
A When you upgrade any Mac with an Apple Fusion Drive to Mojave or Catalina, that drive is automatically converted to Apple’s new APFS file system. You don’t have any choice, and both versions of macOS require to be started up from APFS rather than the old Mac Extended or HFS+ format.
Although APFS isn’t ideal on a hard disk, where most users notice reduced performance, it does work well on Fusion Drives and SSDs. macOS then uses the SSD component within a Fusion Drive to cache active files, and improve the performance of main storage on the hard disk.