21 : Selected disk does not exist
This error is returned if the device part of a device- or full
file name refers to a disk or BIOS device that is not present or
not recognized by the BIOS in the system.
Your IDE listing a Primary master and a Third master, but no Second master is *curious*.
On the back end of the hard drive, there is a set of jumpers with a small (possibly white) piece that can be moved around on different pins (5 sets of two pins per set if I remember correctly). There should be a decal on the back of the hard drive explaining where to put that jumper to make a Master or Slave drive.
You settings of Primary and Third imply that you have two hard drives, each on it's own IDE cable. Is that correct?
If both drives are on the same cable, then they should show up in the BIOS and Primary master and Primary slave. If the two drives are connected to seperate controllers, you would get Primary and Third master.
So, I'd suggest starting with checking the jumpers on the drives.
1) are they on the same cable? If so, set the jumpers to master on one, and slave on the other.
2) Your /boot/grub/menu.lst (grub.conf on some distros) may not agree with actual disk locations. Are you disks called hda and hdb, or something else?
With both drives on the same cable, I'd expect hda and hdb. With each drive on a seperate controller, I'd expect something more like hda and hde (cd and dvd drives would be hdc and hdd).
The menu.lst must point to the correct drive and partition locations, or you will get error 21.
So, check the cables and jumper settings. If you still can't fix it, come back with menu entries from menu.lst, and possibly /etc/fstab.