Error - ''boot_unmountable_drive'' when installing XP#1 - Easiest Way
I have an Asus A7V motherboard and I have run into this more than once. What you need to do (this is the easiest way to do it that I have found) is to move your hard drives cable off the UDMA 100 controller (normally color coded blue) over to the UDMA 66 master controller on the motherboard. Once you have done that make sure your PC still boots into your current OS correctly. If it does then start your XP install or upgrade. Everything should be fine. Now, once XP is up on My Computer and choose Manage. Look under Device manager and you will see an error with a yellow exclamation point on it. Right click on it and install the Promise Drivers.
Once you have the drivers installed re-boot the system and make sure the yellow exclamation points are gone and the promise drivers are listed under SCSI devices, if they are then turn off your system, move the HD's back to the UDMA 100 controller and boot it up. That should fix it.