Which disks get converted by Catalina?

QWould 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?

AWhen 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.

The only potential issue arises with Fusion Drives with SSDs smaller than 100GB, where high levels of disk activity could in some cases wear out the SSD and cause early failure, but so far this appears rare. In contrast, Time Machine backups are still required to be on disks which are formatted using HFS+. So your current backup disk won’t have its format changed. However, Catalina in particular makes changes to the structure of existing backups which prevent you from accessing them using older versions of macOS and Time Machine. Your first full backup after upgrading can also take many hours or days. Consider archiving those old backups and starting a fresh set for Catalina, which can save time and trouble.