By hard disk, I mean any user storage space available to you, usually somewhere in your home directory.
And drive letters are a windows thing. Linux doesn't use them. Physical hard disks can be mounted anywhere on the file tree. At the very least you'll have /, your root directory, on one physical partition. If you have multiple partitions or drives, then you may have various sub-directories, such as /home, actually existing on them instead.
The "mount" command without any options will show you all the drives your system is currently using and their locations in the file tree.