[dba-Tech] Operating System not found

I am replacing the system disk in one PC with a larger one.
I have the original disc intact
and I have installed the system on two other disks now and had it running. 
(made an image of the original disk and restored it to the new disks by 
running them as slave).
Now all of the sudden I get

Operating System not found

when I try to start it with either one of these disks.
when I try to run the PC, I have now only one disk as master connected, and 
yes the power cable is alse connected and the disk is spinning.

Now I started the PC from an Arconis disk, and there I can see the System 
disk with all the data on it, so it cannot be the cable.
What else can suddenly fail such that I get this error?

Thanks
Lembit