Back up disk Time Machine

I have been using cheap (usually, free) hard disk drives pulled from Windows PCs installed in inexpensive enclosures. I don't handle them with any particular care. One recently failed after being generally mishandled for about twelve years.

The LaCie mobile hard disk drives are good.

The most reliable backup disk is more than just one backup disk.

Western Digital, Seagate, Toshiba, Hitachi / HGST all manufacture equally reliable or unreliable hard disk drives. Since any storage device will fail eventually and can fail at any time, you need to have at least two.

SSDs are fast but speed is not a factor with Time Machine. Simplicity and reliability are. Also, for what it's worth, hard disk drives often exhibit signs of failure long before they fail completely. Sometimes, years in advance, during which time they continue to work. They may not work very well in that state of failure though. When Time Machine detects a failure it "locks" the last complete and intact backup, and lets you know.

On the other hand when SSDs fail, they generally do so catastrophically with no warning. The only SSD failures I experienced gave no warning at all. They just stopped working.

I always use a drive with my own enclosure. I have switched to all SSD's for speed and to reduce clutter on my desk. I use WD and Samsung drives mostly.