Disk failed in My Book Duo - Seeking clarification on RMA process

Hi folks,

Got a 6TB My Book Duo I purchased in November 2019 connected to a late 2015 iMac that started flashing its amber light and WD Drive Utilities telling me one of the disks has failed. I’ve got the failed drive out and both it and the Duo unit registered with WD.

Am I right in thinking for the RMA I should do it using the hard disk’s details as that is the component that’s actually failed?

Also can anyone in the UK who has done an RMA post Brexit advise on the turnaround time these days?

(I’ve seen posts on hefty customs charges for replacement drives to enter the UK and I hope I’m not going to get stung too much there.)