Hitchhiker's Guide to Openbsd

ERR R -- BIOS returned an error when trying to read a block from the disk. Usually means exactly what it 
says: your disk wasn't readable. 
●
ERR M -- An invalid 
magic(5)
number was read in the second-stage bootloader's header. This generally 
means whatever it was that was read in was NOT 
/boot
, usually meaning installboot(8) was run 
incorrectly, the /boot file was altered, or you have exceeded your BIOS's ability to read a 
large disk
. 
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14 - Disk Setup
Other error messages are detailed in the 
biosboot(8)
 manual page. 
For more information on the i386 boot process, see: 
●
boot_i386(8)
 
●
http://www.ata-atapi.com/hiw.html
Hale Landis' "How it Works" documents. 
14.8 - What are the issues regarding large drives with OpenBSD?
OpenBSD supports both FFS and FFS2 (also known as UFS and UFS2) file systems. FFS is the historic 
OpenBSD file system, FFS2 is new as of 4.3. Before looking at the limits of each system, we need to look at some 
more general system limits. 
Of course, the ability of file system and the abilities of particular hardware are two different things. A newer 
250G IDE hard disk may have issues on older (pre >137G standards) interfaces (though for the most part, they 
work just fine), and some very old SCSI adapters have been seen to have problems with more modern drives, and 
some older BIOSs will hang when they encounter a modern sized hard disk. You must respect the abilities of your 
hardware and boot code, of course. 
Partition size and location limitations
Unfortunately, the full ability of the OS isn't available until AFTER the OS has been loaded into memory. The 
boot process has to utilize (and is thus limited by) the system's boot ROM. 
For this reason, the entire /bsd file (the kernel) must be located on the disk within the boot ROM addressable area. 
This means that on some older i386 systems, the root partition must be completely within the first 504M, but 
newer computers may have limits of 2G, 8G, 32G, 128G or more. It is worth noting that many relatively new 
computers which support larger than 128G drives actually have BIOS limitations of booting only from within the 
first 128G. You can use these systems with large drives, but your root partition must be within the space 
supported by the boot ROM. 
Note that it is possible to install a 40G drive on an old 486 and load OpenBSD on it as one huge partition, and 
think you have successfully violated the above rule. However, it might come back to haunt you in a most 
unpleasant way: 
●
You install on the 40G / partition. It works, because the base OS and all its files (including /bsd) are within 
the first 504M. 
●
You use the system, and end up with more than 504M of files on it. 
●
You upgrade, build your own kernel, whatever, and copy your new /bsd over the old one. 
●
You reboot. 
●
You get a message such as "ERR M" or other problems on boot. 
Why? Because when you copied "over" the new /bsd file, it didn't overwrite the old one, it got relocated to a new 
location on the disk, probably outside the 504M range the BIOS supported. The boot loader was unable to fetch 
the file /bsd, and the system hung. 
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14 - Disk Setup
To get OpenBSD to boot, the boot loaders (biosboot(8) and 
/boot
in the case of i386/amd64) and the kernel (
/
bsd
) must be within the boot ROM's supported range, and within their own abilities. To play it safe, the rule is 
simple: