Have a look at the hard disk with the tools that come with your OS. You don't write which OS it is, so I'll treat two common ones:

In Windows 98 go into the MS-DOS-command windows and type fdisk /status. Copy the output to a second post. Yes, it can be done, copying from a MS-DOS-command window.

In Windows XP, go into disk management. See http://support.microsoft.com/?kbid=309000
And tell us the results.

Most likely causes:
- It isn't a 30 Gb disk.
- The BIOS doesn't support such a big disk.
- There is only 1 partition of 10 Gb, and 20 GB empty.
These tools maybe will tell.