[jeffdonnelly]

UPDATE:
Ok, an info update after further work...

I tried again with another uSD card.

This time with a Sandisk Extreme PRO 128Gb Micro SDXC I V30 U3 formatted as exFAT.

The first try with this one had exact same results as before in Calibre, with a Comm error thrown. Also, the uSD card had all the directories written to the uSD card by the P78 Pro immediately upon insertion into the P78 Pro e-reader. I subsequently removed the uSD card, and via PC card reader deleted all the folders including the Android and Books folders.

I then recreated a new Books folder on the uSD card and ejected the uSD card from the PC reader and reinserted into the P78 Pro. Upon checking the uSD card after insertion into P78 Pro, I found that P78 Pro had again recreated the Movies, Music, and Photos folders, which i subsequently deleted while in the P78 Pro.

I then plugged the P78 Pro back into the PC running Calibre Portable, and both P78 Pro and the uSD card were recognized without errors. Subsequently, I tried moving books to uSD card via Calibre (Save to Device, Card A) and that also worked as expected. I really believe that having all those folders written to the uSD card upon insertion are causing issues (particularly the ANDROID folder), especially if their are hidden files being written i cannot see or access....

I now have a SanDisk 256GB Extreme microSDXC UHS-I uSD card on order.... It's very strange that the Samsung card will just not work correctly.