Garry, it may help to consider that storage devices are a collection of blocks - "sectors" - each containing collections of ones and zeros. For mass storage devices such as disk drives (and USB sticks) the leading sectors will contain partitioning information, and may also contain boot programs.
- On OpenBSD, the disk partitioning schema is the disklabel.
- On architectures that use MBR or GPT partition tables, the disklabel is used to subdivide the OpenBSD partition defined in the MBR/GPT table.
- If the system is not multibooted, then there is only one entry in an MBR partition table for OpenBSD, and up to two entries in a GPT table, for OpenBSD and for boot, respectively.
- A disk may have up to 16 disklabel partitions, labelled "a" through "p".
- Each partition in the disklabel has a starting sector number, and a size, in sectors.
- Disklabel partition "c" is special. It is always the entire physical disk, starting at sector #0 and with a size that is the number of sectors on the disk. The 15 other partitions are available for filesystems or swap space: "a" and "b", and "d" through "p"
- On architectures that use MBRs or GPTs the first partition will normally never start at sector #0, to leave room for the MBR, the GPT, or boot programs. You will often see the first partition start at sector #64, or #128. or even sector #2048.
This is why
ibara noted your fsck attempt was in error. You chose partition "c", which starts at sector #0, and your filesystems on disk drives should not start with that sector number.