USB sticks are prone to failure. Btw, it is alright to rename it to any format.
Use multiple ones, 1 usb costs about 3-4$ here. You can easily buy 10 of them and then put them in a faraday box to shield against EMP charges.
I dont know about renaming, certain formats have different encoding and it might fuck up the data. Like if you rename a .mp3 to .txt, and then back to .mp3 it wont work anymore.