Lost, need help

Hopefully this is the correct sub, Im no tech wiz so I'm a little lost. I have a DS920+, Ive ran it as a plex server with 2 x 8TB Western Digital HDDs with no issues for a few years.

I recently purchased 2 more 8TB Western Digital HDDs (they are compatible) to increase the storage space and thats where I'm having issues.

Originally I turned off my NAS and installed the new drives in bay 2 and 4. NAS booted up but started making some new sounds that didnt sound great and after a while the NAS wasnt coming online (this may have been where I went wrong) so I shut it down and took the drives out and reinstalled them to make sure it was all sitting correctly. Turned the NAS back on and it booted normally but only the old drives showed up as online as per the lights on the side of the NAS, in the DSM on my pc only the old drives show up and theres no option to add drives in the storage manager, its greyed out.

I tried swapping which bays the drives were in but it only ever registers the old drives. If i hot swap the new drives in when the NAS is running the lights on the NAS light up, showing that it recognises that all 4 drives are installed but still nothing changes in the DSM when I go to add them.

I plugged one of the new HDD into my PC to make sure it even worked and it shows up but if I try to initialize it i get a message saying "The request could not be performed because of an I/O device error" and now I'm worried that I've somehow bricked it as some googling seems to suggest that may mean the HDD is corrupted.

Any help would be greatly appreciated.