More than one backup needs to be saved and I am not very sure about saving it on a USB because there is still a chance that the USB will be infected with a virus which will cause the files to be damaged and no longer accessible.
If it an airgap device wallet, there is no problem. Get a new USD drive and backup the file. If it is on an online wallet, it may not be safe. The problem this can have is if the USB drive is damaged or corrupted which can happen. The user will have to be doing the backup periodically like every 6 months on another USB drive.