What does the sticker on the drive state?
(A quick google looks like they don't need 12V, but the higher capacity drives draw a reasonable amount 1.2A or more on the 800GB+ drives which is more than 2.5" HDD draw these days)
Does your enclosure have a single usb, or does it have a 2nd "power only" usb connector (If so this will likely need to be plugged in)
You need to try it inside a PC, if it works then it's an enclosure issue.
S3500 definitely lists 12V on the sticker so would be surprised if that worked in a USB enclosure.
MX500 is at most 1.2A 5V for the 2TB version from what I can see.