If you are using KDE:
-right click on the desktop and select "create new" "hard disk".
-In General, enter the name you wish to give it.
-In the device tab:
-give the device file (in your case, /dev/hda1) in the device field.
-give the mount point (in your case, /mnt/hda1) in the mount point field.
-give the file system (ex2 or vfat) in the file system type field.
-Click OK; and it should appear on your DT.
If you're not using KDE, then I'm afraid I can't help you.