When you plug in a USB drive, does Windows offer to encrypt it with BitLocker? If you refuse, are you prevented from writing to the device?
Solution - create
disable_bitlocker.bat with the following content on a single line:
REG ADD HKLM\System\CurrentControlSet\Policies\Microsoft\FVE\ /v RDVDenyWriteAccess /t REG_DWORD /d 0 /f
Save the file and simply run it (dbl-click) before inserting your USB Drive.