You seem to have a lot of removable drives.
- Are these USB devices (USB hard drive, USB memory stick)?
- Do you have a spare device you could use for OpenBSD?
- Does your BIOS support booting USB devices?
If the answer is "Yes" to all three of these questions, you need not touch your hard drive at all. You can install OpenBSD in a USB device, and boot it. USB storage (memory stick or hard drive) are attached as SCSI devices, using
umass(4) and
sd(4).