I had two SSD's - when I tried to use them for time machine backups, they were automatically formatted as APFS without giving me an option to select Mac OS Extended Journaled.
The problem is, there is some kind of bug in Big Sur, where randomly after a few backups there is an error during the "Preparing Backup" stage, where it just says there was an error (no detailed information). This has been heavily documented on Apple forums.
I have found that there is nothing wrong with the drives, and they work fine when formatted as Mac OS Extended Journaled. But Big Sur does not give me the option to create my time machine drive with HFS+. It automatically chooses APFS, and reformats my drive to APFS if I format it to HFS+ first!!
At first I thought this was just for SSD's, but now I find it is doing the same when I try to backup to an ordinary HDD.
How can I prevent this from happening? This seems to be some bug in Big Sur Time Machine backups. I cannot update to Monterrey because I have an old Mac which does not support further updates.