I have an Asus P5VD2VM mobo. I have a 3,5" floppy drive connected at A:. How do I connect a 5,25" floppy drive at B:? The system BIOS does not seem to contain that option.
Its not very likely you can - A: and B: being reserved are a historical artifact, and I do distinctly recall, granted this was a very long time ago that when you had more than one 5.25 inch floppy drive, each had a separate controller card (this was pre IDE).
What I don't remember is if they used a standard floppy controller of the type you would find on a 'modern' floppy.
In addition your motherboard at most would have a single floppy drive connector (which the manual confirms). As is, you would have nowhere to plug in a second drive - and as such it shouldn't show up in the system bios on a vanilla motherboard without a controller card.
If you had a suitable card to run additional floppy drives, it will very likely take care of the boot related changes needed.
Finally A: and B: is a dos/windows/cpm(maybe) ism. The OS should handle that.
Quite simply, running a dual floppy system without additional hardware to take care of the grotty bits isn't something ASUS thought was likely enough to have an option for in the hard drive.