I have a compiled a custom 3.10 kernel with just the essential drivers in it. I also have a Centos initramfs with the basic initial file system to go along with it. It boots fine and I can get the networking up and running. I intend to work in this embedded type of environment so I do not need any additional utilities.
I have used "devtmpfs" to populate my /dev directory and I can see all my /dev/sda partitions there. But when I say "mount -o ro /dev/sda1 /mnt", I get an error: No such file or directory.
I built the hard disk SATA drivers (libata) into the kernel. I'm not sure if I need something else too.
I have a very simple init script, exactly the one shown here : http://wiki.gentoo.org/wiki/Custom_Initramfs . Is there any chance I overlooked something? Am I missing any crucial drivers? It's driving me crazy that something so simple has me stuck for such a long time.

I think this seems more likely to be a thread for the centos forums, but does /mnt exist?

Yes, I created the /mnt directory in the correct place.
I feel that this question is distro-agnostic and it has to mostly do with creating a minimal Linux from scratch, hence it would be better in this forum.

Non-intuitive workaround again!
My disk filesystems are ext3. I compiled the ext3 and ext4 drivers into the kernel and executed "mount -t ext3 /dev/sda1 /mnt". It gave me the error : "no such device".
Then I compiled the ext2 drivers in as well just to be sure and executed "mount -t ext2 /dev/sda1 /mnt" . To my surprise, it mounted the disk in ext2 and gave a warning "mounting ext3 filesystem as ext2".
Not really a solution but it gets the job done.