Hi Peter,
Just curious is this a W7 OS installation USB? Like a W7 boot disc or Recovery Disc? Or are you running W7 entirely from the USB stick each time you boot your computer? If so, is this for testing purposes or what? The answer for you lays in the way you are running Windows; windows was not designed to run live from external media-ALWAYS! It is used to repair Windows or to completely reinstall it in case of disaster such as when a nasty Ransomware virus locks you out of your computer or locks and encrypts all your files. Ordinarily in PCs, the BIOS is designed to run Windows (any version) from some kind of any internal hard drive (HDD, SSDD, SHDD). You question leads me to believe you are not running Windows in the manner...