Approved USB device is triggering a block after entering the Bitlocker password, but the device is accessible for write after acknowledging the Bit9 notifier.
Removable device setting on the policy has Active to block Write action on Unapproved Removable Devices. Ntoskrnl.exe gets a block when using Bitlocker on an approved USB device.
Create a custom rule
Rule type: Advanced
Write Action: Allow