can I use any external hard drive to back up iMac 2010?

Yes, you should be able to use that external drive with Time Machine(or manual backup if you wish)

Take a look at this article:

Back up your Mac with Time Machine - Apple Support

Good luck!

Your WD drive should work assuming it is already formatted for use on your Mac. However most experienced users like to use higher quality drives than the WD or Seagate enclosures you find in big box stores. What most of us use are the OWC Mercury Elite Pro series, due to their high quality, the service from the company and how long the enclosures last.

If you want info on those drives please visit https://eshop.macsales.com/shop/owc-mercury-elite-pro

If you mean "backup" by using Time Machine, the typical recommendation

is a drive that is 2-3 times the size of the internal drive.

If you are cloning your internal drive, you need a drive the same size as your internal.

Beyond that, any drive will work if formatted correctly.

On a 2010, since it only has USB 2.0, any large amount of data will take a very

long time to backup and restore. However, any sort of backup is better than no

backup.