Many years ago, I set up an external USB HDD as a Time Machine volume. I don't know whether I did anything special at the time (and this answer indicates I didn't), but I've been using it as both a regular external HDD to store stuff, and as a backup volume (
Backups.backupdb/ folder) all this time.
When I tried to set up a new external HDD that I recently bought, it looked like it needs to be a dedicated HDD for Time Machine only. Time Machine Preference Pane did not allow me to use the existing volume, but insisted on deleting the volume and starting over, and then Finder showed it as read-only even outside of
How can I set up an external HDD as Time Machine backup volume while allowing me to use it as a normal external HDD?