I would get a USB drive, most of them are USB Mass Storage Devices.. standardized.. so OpenBSD should recognize them, perhaps it won't be able to utilize any of the software controlled buttons on the name branded models, but that isn't important.
It is possible to find a quirky device that doesn't work, but most stores are refund policies.. so.. you should be able to find a replacement in that case.
FFS predates widely available portable mass storage devices, so all on-disk structures are in the hosts native byte order..
Little Endian vs Big Endian.
That means you won't be able to mount your partition on a i386 system, or any other system with hw.byteorder as 1234, only systems where hw.byteorder is 4321.
If you don't plan on mounting this drive on any other system, there isn't much of a problem.. except if this system suddenly fails, you'll probably have to find another Big Endian architecture to retrieve your files.
Possible solutions are to use EXT2 on the drive instead, OpenBSD supports mounting this file system in the GENERIC kernel.. but you'll need
sysutils/e2fsprogs to create your partitions.
A network storage device might be another option, some exist that provide NFS/SMB/CIFS.. or just FTP.
I hope that helps, good luck gosha.