I would suggest that the My Book external drive be accessible only by (Windows) users who enter the unlocking password.
At the moment, and as confirmed by Western Digital Service and Support (October 2015), when one (Windows) user unlocks the My Book external drive, that user but also all the other users gain access to the drive. There is no option for restricting access to the drive only to those users who enter the unlocking password.
This presents a serious confidentiality and security issue, since users who don’t know the unlocking password can gain access to the drive if a previous user unlocked it and did not unplug the drive or shut down the computer after use, even if that user who unlocked the drive logs out.
I am using a My Book 4TB with Windows 10.0 (Build 10240).