I have dual 2TB Sandisk Extreme Pro SSD's that have the fastest USB-C revision. Actual drives inside are Samsung. Just about any USB-C SSD will be OK. Just make sure it supports the fastest and most current USB-C protocols. Thunderbolt is going to be

One is running the system on my iMac since the internal Fusion drive went bad bad. The other runs a full backup with daily update using Carbon Copy Cloner.

The main drive also backs up to a Time Capsule through Time Machine.

I switched to this setup after a system update went bad and caused my iMac to run super slow with lots of pinwheels. And the Time Machine backup was corrupted due to the corrupted internal drive on the iMac. Luckily I was able to recover everything by creating a new user account on the iMac, then moving things between the old account and new account, then to the external drives.

As an alternative to an enclosed hard drive, I have found using "raw" drives with a SATA adapter to work quite well for my backups.

Something like:

https://www.amazon.com/StarTec...23410529&sr=8-4&th=1

with a WD Black or Red 2.5" hard drive is what I use.

This makes using a plurality of hard drives easy to use and interchange.

If you want to use a 3.5" hard drive, I recommend getting an adapter with an external power adapter. But also, if you get an adapter with an external power adapter, you can also use a "raw" SATA disk drive. For some important backups, I burn things to M-Disc Blu Ray Disks.