Is this HD installed in a PC, external HD enclosure, NAS (network attached storage device)? Do you need FTP access only locally (i.e., within the confines of your LAN), or remotely as well (i.e., from the Internet back into your home/office)?
If the hard drive is installed in a PC or external HD enclosure, then you would need an FTP server installed on the same PC configured w/ access to the hard drive. Most NAS devices already support FTP as an access method (i.e., they come w/ a built in FTP server).
As far as client access is concerned, that can be everything from simple to complex depending on whether you do or don’t need remote FTP access. FTP might not even be a good choice, there might be better options. Again, more information would help.