Formatting a USB Flash drive is completely safe, assuming you dont have anything important on the flash drive. If you were planning on having an image on the flash drive + retain personal data on the flash drive, then you will need to copy whatever data is on this flash drive to a safe location such as a folder on your hard drive named USB_Data or something like that, and then format it the USB drive after that as for formatting will wipe out all data on the USB drive in the process of formatting and making it NTFS format. Most people using a USB drive for storing images dont have other data on the USB flash drive and so this is not a problem, however I have seen before people have 1 USB drive for everything and then accidentally delete their data from it so trying to help you avoid this if that is the case.
Additionally to create a image to USB flash drive the USB flash drive will need to be equal to or greater storage capacity than the drive data that is being imaged, so if you have a 1TB hard drive and there is 173GB in use of the 1TB hard drive, you would need a USB Drive that is equal to or greater than 173GB. *Note some utilities compress and so it might squeeze it down to 159GB etc, but its a requirement to have a drive that is large enough for the data that is being stored.
If your creating a factory recovery set using a utility that came with the computer to make this media, the size of the USB flash drive is dependent on the requirements for the computer, such as I have a netbook that requires a 8GB USB flash drive and a Laptop that requires 2 QTY 8GB USB sticks and a single 16GB or greater wont work. Running this utility it will instruct you what you will need for media storage sizes and how many etc.
*For most image backups of a system people find themselves requiring a USB attached external Hard Drive such as a 500GB external USB hard drive such as in the example above with 173GB of data to be imaged to it.
If your creating an image to a USB flash drive I am assuming its a large capacity USB stick such as 64GB or 128GB and your data to be imaged to this drive is lesser than 64GB or 128GB in relation to the USB drive.
Have you verified that the USB drives capacity is adequate for the image creation process on the USB drive, as for most people use USB external hard drives vs USB flash drives for this process?
I am assuming your using the backup utility feature built into Windows and its not Macrium Reflect or some other 3rd party software?
