Using boot0 to redirect booting to another disk?

[Warren Block]

On Wed, 3 Dec 2014, José María Alcaide wrote:

> I have an HP Proliant Microserver Gen8. Nice machine but picky BIOS. 
> When its hard disk controller is configured in SATA AHCI mode, there 
> is no way to choose the boot disk among those connected to the SATA 
> ports. I have four HDD attached to the backplane, and another drive 
> connected to a fifth SATA port originally intended for an optical 
> drive. The four HDD are arranged in a RAIDZ. Currently the machine is 
> booting from an USB flash device, but I would like to boot the FreeBSD 
> installed on fifth drive. The disk controller sees and reports the 
> five disks just fine, but as I said above, there is no way to choose 
> the fifth disk as a boot device.

Because the BIOS assumes that fifth drive is always a CD?  And 
presumably that fifth connection is lower bandwidth so switching one of 
the ZFS drives and the boot drives could impact array performance.

> I wondered whether I could use boot0 to redirect the boot from a USB 
> flash device (pendrive or, still better, a microSD) to the fifth 
> drive. The idea comes from the fact that boot0 shows a "F5 - Drive 2" 
> option when it detects more than one drive. I tried to understand how 
> boot0 works reading its source code, and I experimented with the 
> boot0cfg's "-d disk" and "-o setdrv" options, to no avail. So I 
> decided to ask for help. :)

boot0 is very limited, and it is not required to stick with only FreeBSD 
utilities on the USB drive since it is just loading the boot manager. 
Consider one of the more capable boot managers like Plop, SYSLINUX, or 
Grub2.

http://www.plop.at/en/bootmanager/index.html
http://www.syslinux.org/wiki/index.php/The_Syslinux_Project
   or sysutils/syslinux
https://www.gnu.org/software/grub/
   or sysutils/grub2