I needed to test something in OpenBSD. So I’ve installed it on a separate partition on my AMD-based laptop. What a surprise – when I wanted to boot up the other system (Solaris), it turned out to be quite difficult.
In OpenBSD, if you want to choose (switch) the active (bootable) partition, you can’t do it on a hard-drive you’ve normally mounted from (read/write mount). You need the hard-drive to be mounted read-only. For that you need to start in a single-user mode. Right after you turn on/reset your computer, when the OpenBSD boot screen shows up, type boot -s. Then it asks you for terminal type and it doesn’t give you any default or common choice(s). It looks that vt100 is a good one. Then you boot up in a single-user mode.
You can run fdisk and it just prints the partition table and exits. Now let’s go change things: run fdisk -e wd0 where -e stands for interactive mode. You can type manual to show the commands at any time. If you want to change the active partition, type flag numer-of-partition. To commit the changes type write.