Here is my problem: my mini-SD card used to work fine in my PN-820. I really don't use it very often though.

Some time ago I noticed that "\SD Card" folder (or whatever it was called) is no longer visible in File Explorer YET AT THE SAME TIME the icon on the top of the screen that indicates the card is inserted is visible.

Applications that rely on files copied on the card no longer work. Removing the card makes icon on the top of the screen disappear as expected. Inserting card back makes icon appear again but the "\SD Card" folder is not mounted. Only once after insertion SD card was visible for a short period of time but very soon disappeared again (I started application that reads files from the card, it hanged, I restarted my phone and the card was no longer visible).

Anyone had this situation before?

I had the same issue come up. I put the card in a card reader and couldn't access it. I had to buy a new card and I have no problem now.

This just happened to me the other day... I took out the card and cleaned the contacts, then blew out the cavity where the card is inserted and am back up and running.

The PN-820 will show the card icon on the main screen regardless of whether the card is functional. I found this out by placing an empty micro-SD/mini-SD adapter in the slot.