Aaarg, frustratingly slow

Have a 1Tb passport ssd. It simply refuses to work with my 2017 iMac regardless of which port I use. When it does mount, it can take minutes to get recognised by the OS. Then transfer speeds are slower than USB 1 and more frequently than not, the drive mysteriously disconnects.
On my 2013 Mbp, it works (relatively) fine although the WD software is an abomination.
What gives?
Anyone else have this kind of trouble with the passport ssd? I want a refund!