Not seeing External HD

New Imac user from PC so struggling. Tried to connect my Sumodisc (500GB USB-2.0 UK) with data for transfer from old PC - where it worked perfectly. External HD just not registering in Finder. Help - any suggestions on how I get it to work so I can transfer all my files? Thanks

No help!

Well that's been useful. Surely someone knows how to get an external hard drive to work on an Imac!

What on the Imac or do I download via the old PC (now no security as XP) and imprint on the external drive?