Instructions to repair or replace HB-F900 Floppy Drive(s)?

From what I hear you can use a PC drive that is modified for MSX that includes also the RDY signal. Although if just for dumping one without RDY may work as well. I have tested some MSX converted drives in F700 and the drive works but on boot into BASIC it freezes the machine until you insert a formatted floppydisk. Booting a game or DOS works fine.

You need an MSX with a WD2793 floppy drive controller (FDC) and a connection scheme (how the FDC registers are mapped into memory) that is supported by the DMK Creator program.
The drives themselves are always OK, as long as they work normally on the MSX. So if you got a working drive on your HB-F900 it will almost certainly work with DMK Creator.

Most (if not all) Panasonic machines have a TC8566AF FDC, so they are not suitable. Sony machines have the WD2793 and the (common) connection scheme is apparently supported by DMK Creator.

Yes, it does. To my knowledge, all of the Panasonics have that FDC, alas. (But I was a bit careful as I'm not 100% sure that perhaps some very uncommon and unemulated machines may have another one.)

Anyway, as I said in the other thread, it should be very easy to replace the disk drive of your Sony, just try to get a few of these drives and try them out Perhaps the first one already works, like on my G900P back then!

If the drive in the HB-F900 is the same as the HB-G900P (which is likely), it's just a standard 34-pin shugart connector that will fit on any standard 3.5" disk drive. That's why I think it is probably very easy to replace.

@luppie: the FS-A1GT doesn't need a drive replacement, but his Sony HB-F900 does

You can try it out safely. Just open the machine and connect the power and the 34-pin connector and try it out.
I have no experience what happens with/without ready signal, but someone else said something about it earlier, I think.