> What can be the problem.

   That's a question?

   I know nothing, but I'd guess that RedHat 6.1 supports your hardware
(disk adapter, PCI infrastructure, ...), and RedHat 6.0 does not.  You
might be able to find the required drivers and install them with 6.0,
but why not simply use the version which works?  6.0 to 6.1 sounds (to
me) like a pretty small step.

   I recently had a similar problem (with SCSI disks) while trying to
install Debian 6.0.x on a Dell T7400 workstation.  The 7.1.0 installer
could see the disks.  I installed 7.1.0.