@Danramos
1) You are right, I did not unmount the card from the Tab. Didn't think of unmounting at all - was used to the NITs that auto unmount the card when opening the back.
2) The microsd card was used in a flashcard - the r4i-gold. Maybe my incorrect unmounting damaged the filesystem. In fact, I had to fsck the card and also erase some files that were still making the ds stall when selected, and then reinstall the firmware.