APFS vs Mac OS Extended (Journaled) for external HDD

APFS gives several benefits over exFAT, for example:

• Protection against meta-data corruption caused by for example sudden power loss, system crash or if disconnect the external hard drive without unmounting it first. exFAT only detects corruption using checksums, but cannot rollback like APFS.

• Better support for full disk encryption

• Allows snapshotting the file system (i.e. you can create "frozen" versions of everything on disk, which will never change - even when you continue to alter the contents of your files)

• Allows cloning a file into two copies that can be independently changed while only requiring the disk space for one file plus the size of the differences compared to the other file

On the other hand, exFAT has a range of advantages too:

• Older file system that is "tried and tested" for years on macOS

• Simpler file system with less overhead

• Most probably slightly faster (but this will depend on the final implementation in the release version of High Sierra)

• As you mentioned, the drive can be used without extra drivers on Windows

Only you can decide which set of benefits are most important for you.

The external drive used to store Time Machine backups must be formatted with HFS+, also known as Mac OS Extended (Journaled). If the external drive is formatted with APFS, it cannot be used for storing Time Machine backups.

The internal drive in your Mac used to contain the macOS operating system can be formatted with either APFS or HFS+. Both choices will support backing that computer up with Time Machine.

If your computer was automatically converted to APFS with the upgrade to High Sierra this has nothing to do in itself with your Time Machine problem. Such a system is still supported to be backed up with Time Machine.

If you want to reverse the automatic conversion from HFS+ to APFS, you'll need to backup the system, reformat the computer with HFS+ and then restore from the backup (or reinstall a new macOS operating system if you haven't got any data to save). Unfortunately there's no automatic conversion program from APFS to HFS+.

Note that such a conversion from APFS to HFS+ does not in itself improve your Time Machine situation.