I was recently upgrading one of our large storage servers to a newer version of Windows and came across a really strange error during setup: "A required CD/DVD drive device driver is missing". This was really odd, since I've installed with no problems to this same class of hardware many times (and even this exact piece of hardware). Even stranger, the error popped up regardless if I was using the real DVD drive in the machine or our KVM's "virtual media" feature (which looks like a USB-connected DVD drive to Windows). After lots of searching and trying various things, I remembered what was different about this machine: it had about 30 storage LUNs on it for all the various disks. Hitting the "Browse..." button in the driver select dialog confirmed the problem- Windows was helpfully automounting all the LUNs and assigning drive letters to them. Since there are more than 26, it ran out of drive letters before getting around to mounting the DVD drive. Setup assumes if it can't find the DVD drive, it must be a driver problem, hence the misleading error message. I always disable automount on our storage servers anyway (since we mount all those storage LUNs under NTFS folders, not drive letters), but you can't do that for setup without altering the boot image. The solution in this case was to hit Shift-F10 from the setup dialog to get a command prompt, then use diskpart to unassign D: from a storage LUN and reassign it to the DVD drive.
list vol
select vol X
(where X is the volume number in the list with D: assigned)
remove
(removes the drive letter)
select vol Y
(where Y is the volume number in the list that is your Windows Setup DVD)
assign letter=D
Once the setup DVD has a drive letter, you can close the command prompt and proceed with setup normally.
