If you are sharing a hard drive between OpenBSD and Linux, install OpenBSD first. Both Linux and FreeBSD can recognize OpenBSD partitions and will easily work around them.
Linux can read OpenBSD file systems, if you have a Linux kernel that supports BSD disklabels. Similarly, OpenBSD can read EXT2FS file systems. OpenBSD also recognizes file systems from FreeBSD 4 or earlier, and FreeBSD recognizes OpenBSD file systems. If you want to dual-boot FreeBSD 5 or later with OpenBSD, you need to create your FreeBSD partitions as UFS1. OpenBSD does not support FreeBSD’s UFS2. In any of these combinations, you may have to edit the OpenBSD, Linux, or FreeBSD disklabels to include the sector information for the other operating system partitions to actually be able to mount those partitions, however.
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