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.

Did you hard-reset it (or better still, clear its nonvol memory)? If that doesn't help then re-load the cure fw and format it again using RMPrepUSM. Create an empty folder in the root named Garmin, load original fw 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).