U drive will not mount

I am running SCO 5.02 and Informix 6 on a Compaq server.  When booting I
get 
the following message " fsstat:/dev/u needs checking
mount: possibly damaged file system, mount failed: no space left on device

{error 28).  when I run fschk it will not choose the /dev/u as it is not 
mounted and I cannot mount.  there looks to be room on the u drive. What
do 
I use the check the u file system?  Do I do this in Sigle User Mode?  What

would the actual command be including switches?

Thanks, in advance.

Ron