Oh, ok. I am not sure for the ones that came with discs, if they are physically identical to the diskless model (they may be). I thought you couldn’t and weren’t supposed to change the discs out for the ones with discs. I have the diskless model and had no trouble putting in one drive (only put one so far) and taking it out as well. Not sure if the bracket for the diskless model is identical to yours…I suspect it might not be.
If you have a problem with one of the drives, I think they come with warranty and can be sent back to WD. But if you were trying to do an upgrade, I am not sure if they were designed to be upgradable. I could be wrong.