when it's connected but disappears, can you see it in disk management? you can search in the start menu for partition to get to the management console. it'll be called "Create and format hard disk partitions". could try a few things here if you can see it.
maybe try it in another windows machine.
could be something wrong with it internally. if its older or got bent at any point, the solder could be loose between the usb head and the board. i've seen that cause issues like this. hard to tell without opening it up and looking at it though.