This is a strange one, simply because
That drive with 'C' is I beleive in the formware of the motherborad and is set in stone so to speak and as for 'B'[5.25] iit haslong gone.
You could try this open the machine and remove the data connector - large flat ribbon from the drive.
Shut the machine down and restart
It should come up with no 'A' or 'B' assumin I have read you aright and you do not have a 'B'
the machine must be very old if it has.
Now reconnect and see if it reestablishes the correct order.
Wait her more anwers coming of that I am sure