The hd* devices are block devices to access MFM/IDE hard disk drives in raw mode. The master drive on the primary IDE controller (major device number 3) is hda; the slave drive is hdb. The master drive of the second controller (major device number 22) is hdc and the slave is hdd.
General IDE block device names have the form hdX, or hdXP, where X is a letter denoting the physical drive, and P is a number denoting the partition on that physical drive. The first form, hdX, is used to address the whole drive. Partition numbers are assigned in the order the partitions are discovered, and only nonempty, nonextended partitions get a number. However, partition numbers 1–4 are given to the four partitions described in the MBR (the "primary" partitions), regardless of whether they are unused or extended. Thus, the first logical partition will be hdX5. Both DOS-type partitioning and BSD-disklabel partitioning are supported. You can have at most 63 partitions on an IDE disk.
For example, /dev/hda refers to all of the first IDE drive in the system; and /dev/hdb3 refers to the third DOS "primary" partition on the second one.
They are typically created by:
mknod -m 660 /dev/hda b 3 0 mknod -m 660 /dev/hda1 b 3 1 mknod -m 660 /dev/hda2 b 3 2 ... mknod -m 660 /dev/hda8 b 3 8 mknod -m 660 /dev/hdb b 3 64 mknod -m 660 /dev/hdb1 b 3 65 mknod -m 660 /dev/hdb2 b 3 66 ... mknod -m 660 /dev/hdb8 b 3 72 chown root:disk /dev/hd*
chown(1), mknod(1), sd(4), mount(8)