SD Card access is intermittent

First our application. We are using a 4G/64G LattePanda to control a small telescope mount and camera for portable operation in remote locations. So far we are able to connect everything and run through the various processes EXCEPT that the SD card we use for storing images and other data is very unreliable in terms of it being available during a session.

The config:
4G/64gb S70CR200 3.05 LP running Win 10 Ent (not LTSB) Build 1703
BIOS - Core 5.011 UEFI 2.4 3/23/17

The problem:

SD card is inconsistently available. Sometimes on launch of Win 10 it shows up as available drive. Other times it doesn't. Even when it does show up it often mysteriously disappears in the middle of a session.

What we have tried:

The initial LP was purchased with LTSB 1607 - We upgraded to full Ent 1703 but problem persists.
We tried multiple brands and sizes of SD card. Same problem.
There are no error messages in Device manager for Storage devices. Attempting to update driver gives routine already have latest driver.
We have reviewed other forum posts and have not seen anything we haven't tried already with the exception of updating the BIOS.

We have a commitment for this rig at a remote location in two weeks. If we cannot get this to run reliably we will have to resort to returning to a full size laptop.

Any help or suggestions would be greatly appreciated.

Bob