Machine has issues, so pulled the micro-sd card - cannot be read on windows machine, so try to put it back - has fallen into the bottom of the machine.
Is there some trick to installing the new one that doesn’t require disassembling the machine?
Langmuir - this is bad design - things like that should be easily accessible - likewise for things like backup batteries.
I used an air nozzle and a mechanics pickup claw.
You know the thing that you squeeze the end, and 3 little claws come out of the end of the thing?
Gently blow some air around until you can see it. Some guys use a stick with blue tape on the end. Good luck.
Blue tape on claw is the way to go. Being the Linux guy, I’ve built some mad skills retrieving that dropped card. Got pretty good on the fifth retrieval.
unless you have one that leaks then the tape is a flop!!! 
I have only lost it once during the 20 times I have removed it twenty times. I had a hell of a time getting the new etch to work, but I finally got it to work. The air pressure in the cabinet that @holla2040 told me about worked the first time. The fact that my machine has no leaks at this point is a plus. I placed a rag inside under the Orange PI first.
Good to hear, I resorted to the same. I have giant hands and couldn’t get them both in there at the same time. Hence honed my ability to retrieve a dropped card.
I see they make a SD card extenders
Wonder if that would work
Work great, use them in many of my consoles.
Hopefully, you won’t have to touch it again. I have looked at the through-panel type and kept it on the front of the machine. It comes with an 18-inch lead and a dummy SD card to plug into the PI.