CHKDSK is not available for RAW drives

When I run the chkdsk command on my boot drive, I get an error message "CHKDSK is not available for RAW drives." What does this mean, and how can I fix it?

C:\>chkdsk c:
The type of the file system is RAW.
CHKDSK is not available for RAW drives.

I am running Windows 7, 64 bit. I am not experiencing any serious problems with my machine- this was just a matter of routine maintenance. I know I have run CHKDSK successfully on this machine before, but it has been a long time (probably several months).

"The type of the file system is RAW." means that CHKDSK was unable to determine the file system used on the drive, and considers it to hold unknown data. CHKDSK is made for verifying the filesystem data structures, and it doesn't make sense to verify something it doesn't know which way is correct. (Try spell-checking a text in a language you can't read.)

That the filesystem is not recognized means the "inconsistencies" are at a level beyond CHKDSK's expertise. However, the fact that you are able to boot from said filesystem confuses the hell out of me.

One possibility is that there is some sort of malware (or anti-malware) that is preventing CHKDSK access to the raw disk device. Run a RootkitRevealer scan and see if it shows anything interesting. If you have a Windows 7 install disc, try running CHKDSK from its "Repair" mode.

Chkdsk.exe is the command-line interface for the CHKDSK program, which verifies the logical integrity of a file system. If CHKDSK encounters logical inconsistencies in file system data, CHKDSK performs actions that repair the file system data (assuming that the data is not in read-only mode).

This means CHKDSK checks the validity of file systems supported by Windows. This includes FAT16 & 32, NTFS, and exFAT. It does not support any other file system, or raw device.