Just to complete this thread, I'll post what I finally had to do to get my disk working normally again.
I used Administrative Tools / Computer Management / Disk Management to delete the two working partitions on my faulty disk. I ended up with a huge amount of unallocated space.
However, I could not delete the faulty partition. The only option available when right clicking on that partition was "Help". There was no "Delete Partition" option.
So I took a chance and ran up TestDisk_win, selected "No log" and then selected the 500 GB drive.
I then selected "Intel" and then "Delete", which deletes all data in the partition table by writing zeroes to it.
After doing that, I restarted the PC and ran up Disk Management again and the faulty partition was gone and I had my 500 GB of unallocated space back again.
The rest was easy - created three partitions exactly the same as my normal disk, gave them the drive letters I:, J: & K: and I've just about finished copying all the data from the normal 3 drives to these newly created backup drives.
The only problem I had was in getting rid of a large (2 GB) directory in the System Volume Information folder in each of the 3 drives I just created. I think the problem was that I had forgotten to turn off monitoring for those drives, so System Restore was saving stuff in there.
But even after I turned off System Restore on all drives, and then turned it back on again (for only the C drive), the huge folder was still present on each drive and I did not have access rights to them.
So I Googled around and found this command to give me the necessary permissions:
cacls "driveletter:\System Volume Information" /E /G username:F
The MS support page is here:
http://support.microsoft.com/default...;en-us;Q309531
So after using that command for all 3 drives I could get rid of the folder and empty the Recycle bin.
Now both of my 500 GB disks have identical drives and identical free space.
Hopefully, this really is the end of the story.
(and no data lost!
)