When you attach an external drive to a Mac to use with Time Machine and confirm the prompt that appears asking if you want to use it for that purpose, you may then be prompted to erase the drive. Why does that happen?
As noted in previous columns, Apple supports backing up macOS drives in both the older HFS+ (“Mac OS Extended (Journaled)” in Disk Utility) and newer APFS formats. But Time Machine backup drives must use HFS+ for now. It’s a weird requirement on Apple’s part after automatically migrating all Macs startup volumes to APFS during recent macOS version updates (10.13 migrated SSDs; 10.14 migrated hard drives and Fusion drives).
