My 436 card locked up on me not long ago and everything worked except somehow it write protected itself and would not save any changes or make any recordings. The 436 micro sd slot is also extremely difficult to close and lock down. If it is not locked down securely it stands a chance of making an intermittent connection on the contacts and corrupting the card which I think happened in my case.
To make a backup card you should install a new formatted card in the 436 or 536, clear user data under the scanner tab (you will probably have to check the show all drives box, select your card and clear the user data. After you clear the user data you should be able to write to the scanner with your current good program in Sentinel.
I keep a good micro sd card labeled for both my 436 and 536 so if I loose a card I have an immediate backup. The cards do occasionally crap out. My original card is in the 536 but I think the 436 problem is caused if you don't lock down the card securely.