CHKDSK hangs

I was trying to make a full backup with Norton Ghost but halfway the
computer hangs (same result with Acronis..).
I then used chksk /F /R :computer hangs in the fix file systems step.
But: CHKDSK /F and chkdsk /R work fine (no errors..) where /R implies /F
??
SFC did not give any errors and defrag works as well.
What is wrong?
I need to backup the computer!!
Please advise


--
renso

Later I got a message MFT bitmap incorrect.
So I did a hard format and re-installed Vista. Now OK


--
renso