It turns out that DOS' ioctl 0x440F (set logical drive map) was
strangely implemented. From the doc I have, this ioctl is responsible
for setting the logical drive to access a physical drive. This is used
for example, on a single floppy PC when implementing the copy a:foo
b:bar command, where a: and b: refer to the same physical device (the
floppy), and this ioctl is called to toggle the access from a: or b: to
the physical device.
This implies that this ioctl is not responsible for creating the mapping
of a: and b: to the same physical device.
The current implementation was trying to achieve this goal and moreover
it was done in the wrong way :-/
The attached patch removes altogether the support for logical drive map
in int21h (and fixed Myst BTW).