Photos backup device

The easiest is an external drive with Time Machine - especially if your Photos library is on your internal drive.

Since my Photos library is on my internal drive, here is what I do:

Maintain a Time Machine backup

Maintain a bootable clone updated monthly or so

use a separate external drive with only Photo's Library copies. (Since this copies the whole library stupidly, it can take a long time.)

If you have your Photo's Library on an external drive, keep a few other external drives and periodically copy your library to them. Note that like above, this is a "stupid" copy and may take a while, depending on the speed of your drive.

Now that I don't work in a secure area, I can even store a copy at work, in case my house burns down.

I have used about everything. Most name brands should be OK. OWC comes highly recommended. I have had good luck with Sandisk Extremes.

Anything can fail. That is why the Department of Redundancy Department recommends several complementary backups.

For Time Machine, where backups are incremental and don't involve the entire library, speed isn't too important, so a mechanical drive, though slower, is nearly half the cost of a fast SSD, and works fine. In fact, since they're half the cost, I have two Time Machine drives that I swap in and out, just in case one fails. Like Keith Barkley I also make a full copy of my Libraries maybe monthly or so. Because I'm impatient I use a fast Solid State Drive for that.

Since you asked, the mechanical drive I use is this, though I also use 1TB versions:

Toshiba Canvio Advance 2TB Portable External Hard Drive USB 3.0, Black

The SSDs are these, though I also have 1 TB versions, as well:

Crucial X8 2TB Portable SSD - Up to 1050MB/s - USB 3.2 - External Solid State Drive, USB-C

These are portable and USB powered-- for a similar price you can get larger AC powered desktop drives that are cheaper per TB.