I have an old Western Digital Elements External USB HDD (3TB) that that mostly sits on my shelf since I’ve moved my data onto a formal network area storage (NAS). After doing some research, it turns out that these external USB hard drives actually have full 3.5" SATA hard drive inside! All you need to do is pry open the back with a knife or flathead screwdriver and remove the hard drive, and remove the USB adapter. Simple!
My hard drive in it’s original case.
After removing the back, you can see there’s a 3.5" drive inside.
I put it inside my Drobo 5D, at the very bottom, and Drobo takes care of the rest!
Here is the video I viewed on how to remove the drive:
Hope this helps you repurpose/remove hard drives from your external USB drives. Sometimes these external hard drives are even cheaper than buying the drive of the same size itself!