Hi,
I have one of those Amitek External FDD drives, with the labels intact. I also have a Silica systems catalogue from 1995 with a picture of that exact drive.
The switch has 3 functions, off, on and anti-virus. The Antivirus mode stops a write to track 0.
A good thing with the Amitek drive is if the FDD dies, just find any old PC FDD with the data connector in the right place and fit it. The 16V8 GAL converts the PC FDD to an Amiga FDD.
Ian