Won't mount but Disk Utility says it's OK

Try this:

Repair the Hard Drive and Permissions

Boot from your Snow Leopard Installer disc. After the installer loads select your language and click on the Continue button. When the menu bar appears select Disk Utility from the Utilities menu. After DU loads select your hard drive entry (mfgr.'s ID and drive size) from the the left side list. In the DU status area you will see an entry for the S.M.A.R.T. status of the hard drive. If it does not say "Verified" then the hard drive is failing or failed. (SMART status is not reported on external Firewire or USB drives.) If the drive is "Verified" then select your OS X volume from the list on the left (sub-entry below the drive entry,) click on the First Aid tab, then click on the Repair Disk button. If DU reports any errors that have been fixed, then re-run Repair Disk until no errors are reported. If no errors are reported click on the Repair Permissions button. Wait until the operation completes, then quit DU and return to the installer.

If DU reports errors it cannot fix, then you will need Disk Warrior and/or Tech Tool Pro to repair the drive. If you don't have either of them or if neither of them can fix the drive, then you will need to reformat the drive and reinstall OS X.

I would stay away from using TT. Stick to the standard utilities like Disk Utility that comes with OS X. If Disk Utility cannot repair a corrupted directory problem, then use Disk Warrior.

Are you booting from the Snow Leopard DVD? Do so and run Disk Utility as instructed previously. Select your volume in the DU sidebar:

As you see above I have selected my Macintosh HD volume. In the DU toolbar your should see a Mount/Unmount icon to the right of the Burn icon. If you find your volume is ghosted (unlike mine) then click on what should be a Mount icon to see if that mounts your volume on the Desktop.

If there is still a problem with mounting the drive, then there's a major problem that you can only fix by repartitioning the entire drive which will result in the loss of everything on the drive.

Tech Tool Pro reports a drive has Bad Blocks when it required error correction on Read.

That assessment is nonsense. The drive uses Error Correction when ever a bit or two get messed up, and considering the number of blocks on modern drives, there will always be some that require error correction to be read correctly. That is not the correct measure of Bad Blocks.

Even the termionology they are using in their reports show they no longer employ anyone who knows what they are talking about.

In the days of Performa Macs with 68030 processors and Mac System 7, Tech Tool was a really helpful tool. These days, whomever now owns that trademark has seriously lost their way.

The drive or volume is not mounted. DU is not able to read the drive at all.

The Mount icon is in the DU toolbar:

Disk Utility is the tool that lives in the land between Physical drives and Mountable Volumes. It does not need to Mount a drive to erase it or install a new blank Volume on it or to check it.

It is very unusual that a drive would pass (Repair Disk) but not Mount. You should be skeptical of issues such as cables, enclosure, and power.

It's a false report and should be ignored.

I would not rely on TT as I've already said earlier even before Grant posted similar advice.

I really think the Rigid Disk Block may be corrupted. This holds the partition map and could prevent a volume from being unmounted. Unfortunately, there is no way to fix that without repartitioning the drive which will remove everything. So, before going down this road I hope you have backups or can make a backup.

Drive Partition and Format

1. Boot from your Snow Leopard Installer Disc. After the installer loads select your language and click on the Continue button. When the menu bar appears select Disk Utility from the Utilities menu.

2. After DU loads select your hard drive (this is the entry with the mfgr.'s ID and size) from the left side list. Note the SMART status of the drive in DU's status area. If it does not say "Verified" then the drive is failing or has failed and will need replacing. SMART info will not be reported on external drives. Otherwise, click on the Partition tab in the DU main window.

3. Under the Volume Scheme heading set the number of partitions from the drop down menu to one. Click on the Options button, set the partition scheme to GUID then click on the OK button. Set the format type to Mac OS Extended (Journaled.) Click on the Partition button and wait until the process has completed.

4.Quit DU and return to the installer. Proceed with reinstalling OS X.

5. Download and install Mac OS X 10.6.8 Update Combo v1.1.

The problem, Grant, is that the drive has been improperly dismounted, and now DU cannot mount it. You can repair the drive but it will always tell you the drive is OK even when it isn't. So the use of DU under this situation isn't reliable. I think that TT may have scrambled the RDB and the partition map.

Is it worth trying again after a Power-Off and Restart?

It certainly can't hurt to try. I think the OP has already, but I may be wrong. But at this point if the OP can still use the drive, then it may be wise to back it up before trying anything new. Just in case once it is shut down it won't start again.