[H-GEN] Transferring a RH Linux system from one drive to another

[Campbell]

sigh.. Time for me to pick the humbug collective brain again..

Suppose:
I am trying to transfer a system from one set of drives to another one.

1a: is there a limit on hard drive size for the drive the root partition is 
on?
1b: is there a limit on hard drive size for the drive other partitions are 
on?

2: what would be the best way to transfer a system from 2 small drives, to 
one large drive (partitioned as necessary)

I have tried ftp'ing the system across with a system running on each drive 
(2 separate machines) , and also tar-ing then and untaring, without 
success.  Minimising downtime is a priority, but if no other option is 
available, mounting the new (large) drive and copying (while running on the 
system on the smaller (old) drives is possible.

Is what I am describing possible, or would I have to boot from a separate 
(unrelated) system and copy from one drive to another.  If anyone has any 
experience/tips or can point me to a how-to, I would be most grateful.

thanks
Campbell