Quote Originally Posted by SG7 View Post
Make sure that drone is not inside a fleet before attempting the conversion.
Yep that's it! They were all in fleets...on their own mind you, silly me! All fixed.
However they still say they'll transform into a missile array before I confirm transformation (even though they're mine deployment drones). But once transformed they were actually mine fields. Phew!

I'll submit this point in a bug report shortly.