[CentOS] Unable to mount devices in rescue mode

Booting to CentOS 5.5 works and I am able to login with all my devices 
mounted. When I boot from the install DVD and type "linux rescue" I 
eventually get the message, after attempting to have one of the screens 
mount my partitions in read/write mode:

"An error occurred trying to mount some or all of your system. Some of 
it may be mounted under /mnt/sysimage."

But nothing appears mounted under /mnt/sysimage. Does anybody have any 
idea of why this error is happening ? Should I try mounting my CentOS 
5.5 partitions manually at this point ?