I was watching a 6290M on Ebay recently. Not sure if it's still there...
Bah. I think I had a 6890 watched, which is the PC version. It's probably convertible, but not easily. At least a few resistors to move and I'm not sure anyone's actually converted the 6890. There were conversions for the 6280 and 6880. I also see a 6290 on Ebay, but no 'M' versions at present. They used to be not uncommon.
Plenty of 6280Ms for about $20 available, but unless you're willing to use PATA drives or PATA to SATA converters that doesn't help much. Does give you four drives instead of two.