Yes, I've confirmed the IWM works fine and system will boot from a Superdrive (Sony MFD-75W) on the internal connector (with pin 9 of the cable disconnected - I put in a DIP switch for testing ). I've replaced the Bourns filter with a new one.

So how do I follow up on "voltage sag" and why would that affect MFD-51W but not MFD-75 drive?

I haven't found any guide to thorough checkout of this type of drive. I've been through all the stages of cleaning and lubrication.

I am wondering whether alignment might have got affected somehow, and how I can check/adjust alignment (I've done it often enough with 5.25" drives on older systems).

Rick