openbsd-sd-setup.sh

$ cd path/to/openbsd/image
$ sudo diskutil list
<other hard drives, not important>
/dev/disk2 (internal, physical):
   #:                       TYPE NAME                    SIZE       IDENTIFIER
   0:     FDisk_partition_scheme                        *63.9 GB    disk2
$ sudo diskutil unmountDisk /dev/rdisk2
Unmount of all volumes on disk2 was successful
$ sudo dd if=./miniroot-am335x-58.fs of=/dev/rdisk2 bs=1m
18+0 records in
18+0 records out
18874368 bytes transferred in 2.116192 secs (8919024 bytes/sec)