To the best of my recollection:

IIRC, hdb is what is actually there to see on a MkII as it has both
disks on the same cable (IDE channel) as master and slave (hda and hdb).

The MkI uses 2 IDE channels and the second disk is master on the
second channel, ie hdc.

To simplify life, hdc exists on the MkII too, but only as a symlink
to hdb.

So, on a MkII, if you want to check for a second disk, you do need to
check for hdb (which is where it physically exists) - but you mount it
as hdc (to keep things compatible with the MkI)

/Michael