When you enable BitLocker, there is a period of time where BitLocker is slowly encrypting the contents of the drive in the background. During this time, is it guaranteed that new data written to the disk will be encrypted as it is written? Or is it possible that it will be written to the unencrypted part of the drive, and won't be encrypted until later?
The drive will be treated as a whole, since it's a Disk/Volume encryption (unlike file or system encryption), everything that goes into the disc will be encrypted as it goes in.
Behavior confirmed in Microsoft Tech Net
Credits to Chris Vasselli