Does this mean my imac is dying? only had issues after installing mountain lion:

Question

One of my drives seems to be at zero capacity, however I do not have near enough stuff on it to fill 499.25 gb Model Name: iMac  Model Identifier: iMac8,1  Processor Name: Intel Core 2 Duo  Processor Speed: 2.66 GHz  Number of Processors: 1  Total Number of Cores: 2  L2 Cache: 6 MB  Memory: 4 GB  Bus Speed: 1.07 GHz  Boot ROM Version: IM81.00C1.B00  SMC Version (system): 1.29f1  Serial Number (system): YM************6  Hardware UUID: 781EE1AC-9FE9-5E63-B077-27AF5AA70DC2 gvu:  Available: 321.42 GB (321,417,748,480 bytes)  Capacity: 498.93 GB (498,929,332,224 bytes)  Mount Point: /  File System: Journaled HFS+  Writable: Yes  Ignore Ownership: No  BSD Name: disk1  Volume UUID: 6600AAE4-B4A2-372B-A1B5-A111A4509A9F  Logical Volume:  Revertible: Yes (unlock and decryption required)  Encrypted: Yes  Encryption Type: AES-XTS  Locked: No  LV UUID: 94883655-9AA9-4F15-A3CA-DD186B155895  Logical Volume Group:  Name:  gvu  Size: 499.25 GB (499,248,103,424 bytes)  Free Space: Zero KB  LVG UUID: 5BB41A33-47E6-49FC-87EC-F4D739377BD2 Physical Volumes:disk0s2:  Media Name: WDC WD5000AAKS-40YGA1 Media  Size: 499.25 GB (499,248,103,424 bytes)  Medium Type: Rotational  Protocol: SATA  Internal: Yes  Partition Map Type: GPT (GUID Partition Table)  Status: Online  S.M.A.R.T. Status: Verified  PV UUID: 5CEE58ED-07D1-4D4E-B1F1-FAF9B0C77970
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Answer 1

There is one thing you can do to try to determine what's consuming that space.  Launch a Terminal session, then type this command:
sudo du -d 1 -h -x /
and press <return>.  You'll be prompted for your administrative password, which won't be echoed as you type it.  Press <return> again after you type the password.
The output will show one line for each file or directory at the top level ("/") of your disk.  That will give you a first-cut picture of what's consuming the space.
Here are the things on my system that have a non-trivial amount of space:
975M /.Spotlight-V100
15G  /Applications
17G  /Library
5.8G /private
3.8G /System
37G  /Users
606M /usr
For more details on a particular entry, just put its name after the slash on that command, such as
sudo du -d 1 -h -x /private