This is my (SpaceRat) choice, as it was possible to buy this drive new right away from normal computer stores at least in 2006.
<br>
You need to remove the shield on the bottom of the drive first.
On the circuit board, just below the red+black+white cable to the motor, you will find two soldering bridges labelled DC and RDY, with a zero Ohm SMD-resistor on the "DC" bridge. Unsolder it there and resolder it on the bridge labelled "RDY".<br>
The Samsung SFD-321B is also hardwired to HD-disks (High Density), we will need to change this to DD (Double Density):
Also on the circuit board, right behind the push button, you will find an empty space labelled SW1, next to it is a soldering bridge labelled C16, destroy the connection it makes by unsoldering it and/or scratching away the soldering tin between the two points.
The drive will now "see" all 3½" disks as DD, regardless of their actual specifications, so as long as you only use the disks on the CPC, you don't even need to close the HD-index-hole, though you will still need to do this on your PC.
This peculiarity of this drive also means that you probably don't want to have it in your PC, as you probably wouldn't want to modify your PC drive to DD-onlyjust to be able to exchange disks between CPC and PC.
So before you go and buy a Samsung SFD-321B for your CPC, check your PC first and if there is the Samsung inside, get a new, non-Samsung, for your PC instead, using the existing Samsung for your CPC.