Mounting LVM partitions on external hard disk

Hi, I have problems to mount LVM partitions on an external firewire disk. The problem is that the LVM groups/partitions (?) are not activated when I plug in the hard disk. The only way to solve it is currently to start the YaST partitioning tool which somehow activates them and makes them available to teh kernel. To achieve a similar thing I tried on the command line: vgscan && vgchange -a y and lvmdiskscan && vgchange -a y But none of these work. So I guess there are other commands to activate them? Cheers, Ulrich