File systems are a problem in general. UFS isn't portable either, even to other BSDs with UFS. Brtfs is only on Linux, ext4 doesn't work on BSD except when mounted as read-only ext2 or 3, or something. NTFS is probably the most portable other than FAT, but OpenBSD doesn't have that either. The best thing to do, I think, is to have a power-efficient file server running FreeBSD with ZFS, and then just sharing the files with your LAN, using NFS (or SMB, I guess).