First, check the SMART status (many free tools can do that) ; it's unlikely to be related with a hardware issue, but it's always wise to ckeck it, especially with an old drive.
Then, if using Windows, check the partition(s) with WinDirStat, or Spacemonger as suggested above, or any other space analyzer (there are most likely equivalent tools on other systems).
If you have many, I mean many many
MANY files on that drive, like several millions of small pictures or text files, it could be that the MFT (Master File Table) it taking a whole lot of space (and space analyzers won't recognize it) ; although it's unlikely to be taking 85%.
Also (again if the computer is running on Windows), you may have folders nested so deep and with names so long that Windows can't take their contents into account, as it's limited to 263 characters for the whole path (in this case I don't know if space analyzers can deal with it and correctly analyze the space occupied by folders inaccessible through Windows Explorer, I'd have to test).
Example :
Code:
D:\Dummy folder name\Dummy folder name\Dummy folder name\Dummy folder name\Dummy folder name\Dummy folder name\Dummy folder name\Dummy folder name\Dummy folder name\Dummy folder name\Dummy folder name\Dummy folder name\Dummy folder name\Dummy folder name\Dummy folder name\Dummy file name.avi
=> This .avi file is located in a folder with a 273 characters path length (the whole path being 292 characters long), Windows Explorer can't access it nor calculate its size so when checking the parent folder's properties (if it's the only file in it) it appears as “0 byte”, even though it may be actually occupying 1GB or more. It can be accessed again, and its correct size displayed, if the path is shortened, by renaming parent folders or moving them upper in the directory tree.