Since the problem followed the disk and behaves the same on different OSes, I’d agree there is definitely something going on with the disk. Given you’ve been doing write tests, I have to assume you’ve already saved whatever data may have been on this drive. But if not, definitely make a backup.
You could try doing a SMART extended self-test and see what the values look like after that. However not all drive problems show up in the SMART data.
Modern drives must take serious measures to protect the embedded servo data when performing write operations. If the drive was having some problem with the head assembly or excessive run-out of the spindle (i.e. wobble) the firmware may go into an ultra-careful mode when writing data which could drastically reduce write performance. However this is pure speculation on my part…
In any case, these forums are only for other end-users to offer assistance. Given the lack of an obvious cause based on what you’ve posted so far, I would recommend you contact WDC Support who may have additional things to try/check: