Opening Horizon and the USB isn’t working? Does it say something like “No USB Found,” or “Run Horizon as an administrator.” There’s a simple way to fix this. 
First Backup everything that you want on your flash drive, because everything WILL be deleted. Insert your flash into the computer and make sure you can see it’s files. If it’s working normally but not loading in Horizon, click the Start button. Click computer, then left click on the desired flash drive, then click format. A menu will pop up for formatting settings. under file system, change it to NTFS. Format your device.
Once formatted, eject your flash from the PC and plug it into your Xbox 360. On your Xbox 360 go to system settings, then storage. Go down to your hard drive and transfer content to your flash drive. Because you formatted your flash drive on your computer you will have to reformat the device. Once formatted to your Xbox 360 transfer any content that you want to mod onto your flash drive and head back to your PC.
Plug your flash drive into your PC and put all of the things that you backed up, back onto the flash. You’re done! Now your drive should show up in Horizon once more. Sometimes this doesn’t work, if not for you, do it again until it does.
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