I have 9 external drives. I dedicate each of them to a specific subject: Time Machine, Wildflowers, General Lightroom Backup, as well as other subjects and locations.
This link is to an interesting article about tests done on Toshiba, Seagate and Hitachi drives. I mostly buy drives with firewire, rather than simple USB drives, for speed. But have NOT had much luck with long-term storage on the Seagate USB drives. It’s good to have my experience validated by testing.