Sorry for the late reply! Must have missed the notification.
Specs for the doc state that each slot should be good for up to 6tb.
To recap, situation as of now, is that I recently attempted to connect the dock, using the 5tb and 3tb inserted, along with a spare 500gb 2.5”, to a Pi4 running LibreElec, and suddenly all three were recognised! I assumed that my previous difficulties in getting the trio working together were possibly related to my original 3B+ Raspberry, in some way (incidentally, running RetroPie, as I highly rate its stability when using Kodi).
Very recently, I picked up a brand-new 4tb drive for the project, in mind to replacing the 500gb, and formatted it using a Win10 machine. When all four drives were slotted into the dock, they were each recognised by Windows, and I could move files around without issue.
Then, I tried connecting the dock back to the Pi4, using the 5tb, 3tb, and 4tb, and once again, none were recognised. One of the dock’s four drive indicator lights became illuminated solid red again, which tends to be whichever slot the 5tb is connected to. Bizarrely, when removed, the 3tb and new 4tb are recognised without issue. Whatever this conflict is, seems to be related to the 5tb.
My hope is that as the dock and all drives ostensibly work fine on a Windows machine, that this is somehow fixable on the Pi. Is there anything I can double check?