I believe those two files are used to emulate hard disk.
when you open the OSBOOT.img using isobuster or similar program, you can see the actual contents.
(i.e. pcdos files, ghost.exe ... etc)
I have a machine with pheonix bioses which GBW CD with pcdos doesn't work.
when I tried msdos, it was working ok.
can you try with msdos and check whether it makes a difference?