I bought two Palm Dummy SD cards, one with a notch at the bottom, one without. Both work fine when inserted, no "Processing" message, no Removable Storage line in Settings. However, if I put a MicroSD to SD Adapter card in, without the MicroSD card, it does show the Processing message and in Settings the Removable Storage line is present.
As I have several extra MicroSD to SD adapters I will next try modifying one of the Adapter cards to see what causes the unexpected behavior. I will post the results of my experimenting.