So, just to refresh, the drive with the reported error gets it whether it is in #1 or #2 position?

If that is the case it is what I suspected................Dell are not a charity, they would not have offered a replacement unless they were pretty certain that it was defective.

Just disconnect the cable from the defective drive and do a cold reboot. Please let me know what happens...............in particular the messages.

So, FIRST THING is to go into BIOS setup and turn off fast boot...... that way we get all the sad error messages whilst you are there, turn off power management, have it always on.