I need a time machine backup hard drive for a 21.5 “ late 2013 imac

You won't go wrong with any name brand HDD. The technology is mature and they all perform pretty much equally well.

If you can, get as fast an HDD as you can ... usually in the 150 MBps range. Some will be as slow as 70 MBps. Speed is not all that important for backing up, but if you have a large amount of data, a slow drive will seem to take forever that first time. Even a faster one may seem like it's taking a long time that first time.

Get 2x-3x the size of your internal drive and don't expect to get more than about 5 years from it. It will likely last longer, but this is your backup life and getting a new drive before the old one fails is not a bad idea.

Hi there!

A SanDisk Professional 2TB G-Drive ArmorATD™ - Apple would work well for you! This external disk includes a USB-A adapter cable, so it’ll work with your 2013 iMac.

To use Time Machine, take a look at Back up your Mac with Time Machine - Apple Support.

Jack

I'd recommend against the cheap drives you will typically find at Walmart. A lot of very experienced users have had great success with the OWC Mercury Elite Pro series. They are reasonably prices, cool well which translate to reliability and the company provides great customer service. You can find them in a variety of capacities at OWC Mercury Elite Pro.

By the way, I have some of these that are well over 10 years old and are still working just fine. The hard drives have needed changing but this is simply done with a screw driver in less than 5 minutes when the time comes years down the road. This is something that cannot be done with sealed case drives like the models you will find at a Walmart.