So to re-cap: With Cure fw loaded the device and inserted sd card can be seen by a PC but not with normal fw and files loaded. .................... ====> Yes
Did you hard-reset it (or better still, clear its nonvol memory)? ........... ====> Yes, using RMPrepUSM
If that doesn't help then re-load the cure fw and format it again using RMPrepUSM. ..........====> Did it again
Create an empty folder in the root named Garmin, load original fw again, ........====> Did it again
boot it and then see if it can be seen by PC without any other files loaded (also leave the card out of it for now). ....... ====> SD Card out, latest original fw, only Garmin folder in the internal memory ..... ===> CAN NOT SEE SD CARD BY THE PC