Well, the full story then:
When i hook up the external 5.21" drive to the Amiga 500, everything is working well.
The drive reads and writes perfectly as it should.
On the A1200 (kick 3.1 / wb 3.1) the drive is recognized on boot stage. The drive starts spinning, but the head is not moving, thus the disk is not recognized.
After boot, the drive appears as DF1:

Trying to format the disk results in error message : Cannot write cylinder 0
Again, the A1200 with kick 3.1 but booted with workbench 1.3 (which is used on the a500), the same thing happens: drive spins, head does not move, disk not recognized.
After boot, drive appears as DF1:BAD
Formatting results in error message : Initialization failed, no sector header, try different disk
Yet again, the A1200, softkicked with ROM 1.3, and booted with WB1.3 -- Same result
Drive spins on boot, no head movement.
After boot, drive appears as DF1:BAD
Formatting results in error message : Initialization failed, no sector header, try different disk
Since the drive works on the A500 flawlessly, i should assume that the drive is OK
As for the A1200, i tried 4 Different motherboards, all with the same result. So i think that this cannot be the problem either.
I'm a bit of desperate, and confused.