For those who have struggled with SD probs as I have, the only solution when you've tried the standard fixes seems to be to get a new or used card reader board from ebay? Get em to check it actually works first! Easy to replace. Youtube shows how. Remove sd card! Run finger nail around tablet edge to open. Unplug battery connector. Watch out for static elec. Flick up the ribbon cable locks with your nail. Remove and replace mini board, connecting ribbons before screwing in place.
I had a couple of these tabs from ebay - only one of 3 had no sd probs. HTHs someone.