Should be fine, just don't cheap out on the external drive / cable you will be using. And when you're using something like smartctl you'll know right away if SMART info is passing through your USB for proper testing.
I've done a lot of these type of scans via USB drives, honestly the more annoying part is that some USB drives do wonky things like go into sleep mode within 1-5 minutes which will disrupt any sort of scanning you had going. So with USB drive scanning I usually implement something to keep the drive alive and awake e.g. a simple infinite loop script to write a file every x seconds, or if you're on windows you can also use KeepAliveHD.