With a scsi drive (or raid) the initial block device modules needed to access the root filesystem are loaded in memory. When you recompile a kernel, you also recompile the modules associated with that kernel, so you need to remake the initrd images. With a scsi system, it was one of the things that I had to learn first, because my new kernels never worked unless I booted them off floppy.

Also check out the following URL for more information

http://mandrakeuser.org/install/kupgrade3.html

Hope this helps. If you have any problems or suggestions then please send me a mail.