The GPS unit may have corrupted the card's format somehow, automatically labeled it read-only or the card itself got corrupted at some point in transit and just needed the reformat even though you were able to retrieve the data from it. I had a customer come in with a similar problem once and could not get pictures off his SD card from his camera. I had to eventually use Linux to transfer the files due to the fact that it looks at and reads data somewhat differently than windows does and tends to "see" more than windows.
I recommend you get another card just in case that one fails.
