It's a 1 GB USB HDD, not a flash dongle. And I didn't do anything weird to it. On Linux computer I used cfdisk and then mkfs.msdos, which created the /dev/sdb1 filesystem. I saved some files to it, then moved the disk to OpenBSD computer. The first time I connected the drive, it detected the correct disklabel, which had an 'i' parition instead of that bogus one. Then I disconnected the drive so I could connect my old backup HDD to get files off of it. When I later reconnected the new HDD to OpenBSD computer, it got confused and showed bogus disklabel. I tried to disconnect and reconnect a couple times and the same thing happened. But the dirve was still working fine when I plugged it into Linux computer, and the files I put on it before were still there.
Now I backed up most of my stuff to it over the network, after formatting it to ext4. Not long from now that OpenBSD computer will be history anyway, since it almost never boots up (it's a shitty laptop with hardware problems).