Hello Richard, and welcome to Seven Forums.
To be able to automatically unlock fixed data drives, the drive that Windows is installed on must also be encrypted by BitLocker
Afterwards, you should be able to pick up at step 10 in the tutorial below to right click on the BitLocker HDD, click on Manage BitLocker
, and select the Automatically unlock this drive on this computer
option to do so.
BitLocker Drive Encryption - Internal Data Hard Drives - Turn On or Off
Hope this helps,