Invalid MS-Dos Function

This is a new problem I'm having.
I capture tv shows via a capture card through WMC onto hard drive G. Then I run them through DVRMSToolbox to convert to MPG from dvr-ms. Then I run the MPG file through VideoRedo to cut out the commercials. I then 'save as" to my D drive. All this to get commercial-free mpg files that I can play with VLC Player.

Last night the "save as" would hang after about 30 seconds. Part of the file would be saved an on my D drive, but only the first few minutes of a 44 minute long recording. Now I'm "saving as" to my desktop so as not to lose the cleaned up MPG file. When I try to copy from Desktop onto D I get an "Invalid MS-DOS Function" error.

G is a 500GB WD with about 40GB unused; D is a 2TB Hitachi with about 800GB unused. I'm running Vista x64, and until two days ago this process worked flawlessly. I did a brief search but found this seems to apply when trying to copy protected DVDs. I'm not doing that, just cleaning up and moving fair-use files on my system. Any suggestions?

What's using MS-DOS anyway?

Try running a scan of the drive in question (hardware check) and see if that turns up any errors.

I tried chkdsk on D from the command prompt and got "The type of the file system is RAW. Chkdsk is not available for RAW drives".
However, Computer Management / Disk Management confirms the D drive is NTFS.

Fire up Spacemonger (one of my old favorites) because now the 2TB drive is showing as 99% full. All of what it shows is supposed to be there, but now I've got 830GB (44%) of "unscannable" folders in my D drive. What could this be? How do I fix it?

I'd run a Linux live cd and run some checks from there.

badblocks is a good program to use.