I don't really remember a lot about Technics older keyboards but one trick which often worked in a case like this was to stick some Sellotape over the hole on the floppy (not the write protect hole - the other one) before inserting it. This is because there used to be 2 kinds of disks - Double Density & High Density disks - it may be expecting to read the older type. I have had this problem with my Technics SMAC 1200 (including the same or similar, error message)and that used to work with that.

I appreciate that the drive worked in the other keyboard so, in theory, it should work now but, in this wonderful world of changing technology, anything can happen so give it a try.
Tony