Short answer: you don't. The USB mass storage device can only be used to access pictures (at least as far as I have been able to use one). Long Answer: Don't know how you got a save game file onto the USB device, if it was from a PC then you won't be able to access the file from the Xbox 360 as the two have different file types for games.
file
Title Bar.
file is one of the storage device
Storage and placed in memory
read to the file
copy and paste
Putting documents on your flashdrive (a USB storage device) is as simple as copying any file on your computer to an enumerated USB Flash Drive using drag-and-drop file listings. Alternatively any program that creates documents gives you the option to write the document to the Flash Drive using its "save as" command. Documents saved on a Flash Drive can opened, edited, then saved back to the storage device without the document being saved to the computer's hard drive.
The file systems are helpful to categorise and organise the files in a secondary storage device.
You can get the saved game of Clash of Clans on android after erasing the game file on the device by going to the app store and re-downloading it.
network attached storage for file sharing between computers
A storage device that can be devided into partition and have volume
In a storage device suitable to the type of file.