Using
newfs(8) on sd0
c will overwrite or "format" the entire device, I'm going to guess that's probably
NOT what you want.. you should use
fdisk(8) to create an appropriate MBR partition table.
OpenBSD's
dmesg(8) can help you identify disks and their corresponding device nodes, the kernel uses
sd(4) for a lot of SCSI-like devices, including USB mass storage.
BSD as well as other BSD's have an additional level of partitioning called "disklabels", on OpenBSD, the
c partition is reserved and represents the entire device. When you have foreign partitions/filesystems OpenBSD creates a "fake" disklabel in memory and starts them off at
i.
OpenBSD has a fairly comprehensive FAQ and is constantly improving the system documentation, or
man pages. You should take the time to become familiar with the system.