The beeper sounds when the resistance is below a threshold level. Therefore a good diode will not sound the beeper.

On the diode test range of a typical digital multimeter, a silicon diode should measure around 600 in the forward biased direction and overrange in the reverse direction. It sounds like your D3 diode is open, which would be an unusual failure mode. If it is indeed open, then it won't affect the working of the drive. However, it may mean that there is damage elsewhere on the PCB. That said, if the power pack is a two-pin type, then it would be unusual for the +5V supply to be damaged. This is because the bridge board in the enclosure has a 12V-5V DC-DC converter that is capable of withstanding an overvoltage on its input.

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A backup a day keeps DR away.

I had to read that twice but Im pretty sure I get it.
I will get onto buying a SMBJ12A (or three just in case...) and fit it to the board.
Will let you know of the results.

Thanks again for your help, you have given me hope! (actually a mate of mine - this is his doing!)