10.2. BIOS Drive Numbering

Before you install and configure FreeBSD on your system, there is an important subject that you should be aware of if, especially if you have multiple hard drives.

In a PC running DOS or any of the BIOS-dependent operating systems (WINxxx), the BIOS is able to abstract the normal disk drive order, and the operating system goes along with the change. This allows the user to boot from a disk drive other than the so-called "primary master". This is especially convenient for some users who have found that the simplest and cheapest way to keep a system backup is to buy an identical second hard drive, and perform routine copies of the first drive to the second drive using Ghost or XCOPY. Then, if the first drive fails, or is attacked by a virus, or is scribbled upon by an operating system defect, he can easily recover by instructing the BIOS to logically swap the drives. It's like switching the cables on the drives, but without having to open the case.

More expensive systems with SCSI controllers often include BIOS extensions which allow the SCSI drives to be re-ordered in a similar fashion for up to seven drives.

A user who is accustomed to taking advantage of these features may become surprised when the results with FreeBSD are not as expected. FreeBSD does not use the BIOS, and does not know the "logical BIOS drive mapping". This can lead to very perplexing situations, especially when drives are physically identical in geometry, and have also been made as data clones of one another.

When using FreeBSD, always restore the BIOS to natural drive numbering before installing FreeBSD, and then leave it that way. If you need to switch drives around, then do so, but do it the hard way, and open the case and move the jumpers and cables.