In UNIX, filesystems don't appear on drive letters, they appear in device files. /dev/hda1 might be the first partition of the first IDE drive, for example. These device files are attached to directories on the directory tree with the 'mount' command, usually according to /etc/fstab. The root directory itself, /, is mounted when the kernel boots.

So there's the root partition, which contains files and directories, and filesystems that you can mount upon directories in the root partition.

You can see what partitions are mounted on what directories with df