Tested on OpenBSD 6.3 and 6.4 (amd64)

Find disk name and partition with sysctl(1) and dmesg(1)

Make sure you are using the device name and partition you need, before your destroy all your data run any destructive commands like dd(1).

Disk names

You can address disk/partitions…

by disklabel UID  
d2L6wcn2dlggwqel raw device
d2L6wcn2dlggwqel.c entire disk
d2L6wcn2dlggwqel.a partition a
or by full path  
/dev/rsd1c entire disk raw device sd1
/dev/rsd1a a partition
/dev/sd1c entire disk same, but as a block device
/dev/sd1a a partition

a raw device (or character device) is accessed directly, bypassing the operating system’s caches and buffers.

When to use block devices then?

“Use block dev only for mounting, use raw for anything else"
Otto Moerbeek (@ottom6k)

There are programs like disklabel(8), they accept all kinds of names (/dev/sd0c, or /dev/sd0, or even sd0) and parse them into full path /dev/rsd0c.

# disklabel sd1
# /dev/rsd1c:

Some programs, like fdisk(8), accept some abbreviations, like sd1 or sd1c, but refuse to work with block devices.

# fdisk /dev/sd1c
fdisk: /dev/sd1c is not a character device

Other programs, for example, dd(1), just do what you told them to do.

Don’t repeat at home:

# dd if=/dev/zero of=/dev/sd3 bs=1m

/: write failed, file system is full
dd: /dev/sd3: No space left on device
919+0 records in
918+0 records out
962592768 bytes transferred in 2.466 secs (390316750 bytes/sec)

This dd creates the file /dev/sd3 and fills up your root partition.

Find disks

$ sysctl hw.disknames

sd0 is the device connected first, its DUID is ew9w8ueO9m0t1wda
sd1 is the second and DUID is 66160c68a67e00e6.

These numbers (sd0 and sd1) may change after reboot or pluging/unpluging USB devices, but DUIDs are persitent until you rewrite it with disklabel.

If you are looking for a just connected USB drive, then grep(1) word removable in the output of dmesg(1).

$ dmesg | grep removable | tail -1
sd1 at scsibus5 ...  SCSI3 0/direct removable ...

sd1 is what you are looking for.

Find partitions

Run disklabel(1) with a disk name or DUID.

# disklabel sd1
# /dev/rsd1c:
type: SCSI
disk: SCSI disk
duid: 66160c68a67e00e6
bytes/sector: 512
sectors/track: 63
tracks/cylinder: 255
sectors/cylinder: 16065
cylinders: 1945
total sectors: 31260672
boundstart: 0
boundend: 31260672
drivedata: 0

16 partitions:
#                size      offset  fstype [fsize bsize   cpg]
  a:         31260672           0  4.2BSD   2048 16384     1
  c:         31260672           0  unused

If you are looking for OpenBSD partition (4.2BSD) it’s the partition a.

The entire disk is represented as the partition c (marked as unused, because you can’t create a file system on this partition).

Thanks to Tim Chase, Devin Teske, Mischa Peters, Bryan Steele for reading drafts of this.