Johnny Five says, "MORE INPUT!"
What computer, what drives, what OS's, what disk format?
When you ask a question like that, you MUST tell us all about your PC.
From what you said, all I could say would be if you're trying to get a DOS prompt on a Windows XP or Vista disk, it ain't a gonna happen!!!!
The disks (HD's) are automatically formatted in NTFS format and DOS can't even see them.
DOS could see a '98 or ME drive, but not on XP which came out on NTFS drives.
More info may clear up the cobwebs......