FreeBSD 4.8: can't load kernel after doing "cp -R /" to another disk

[admin]

Hello, everyone. This is the problem: our SCSI disk with FreeBSD 4.8 on it 
has been failing recently, so I copied its root partition to a fresh IDE 
disk with cp -pR and tried to boot from that. Unfortunately, loader gives me 
this:
can't load 'kernel'
can't load 'kernel.old'
and offers prompt. 

I tried almost any combination of the loader command (like 
0:ad0(0,a)/boot/loader) but still the same. The kernels are right there: 
/kernel and /kernel.old. Can't the root partition be copied this way? Should 
their location on disk be hardcoded somewhere? Please help me with 
bootblocks etc. as I'm desperately running out of time.