Witstick cant connect to the memory stick sometimes.

When i tried it for the first time it works. Witstick can read read. I was able to take a picture too and save it to the memory stick. Since my iphone was full. But when i tried to use it the second time. Witstick app cant read the memory stick. Tried plugging it several times to my iphone before it worked again. Im kinda scared to use it as a back up for my iphone. But using it to take more pictures even your iphone is full was good enough for 3 stars. I hope it works well all the time.