If you start out with your root FS on one device and later move it to
another device, basically what you are doing, there are two things
you need to do:
1. Tell the (new) kernel about the change
2. Update /etc/fstab.
I'll elaborate a bit on #1. If you have booted 2.4.25 and you build
and install a new kernel, it will assume that you are keeping the
same root device (hda, in your case). After installing the new kernel
you need to tell it that your root FS is in a different place (sda now).
One way to do this is to add a root=/dev/hda? to your grub menu.lst or
your lilo.conf and reinstall the boot loader, if necessary.
The other is to update the compiled and installed kernel image to tell
it about the change. There is a command line tool that allows you
to do this, but I can't remember what it is.
Naturally, don't forget update /etc/fstab so you don't have problems
with the rest of your filesystems.
-Roberto Sanchez