Why does BitLocker Drive Encryption use up most of the free space on my disk during encryption?

A. BitLocker Drive Encryption is designed to protect the enabled volumes' data, but when you delete data from a disk, you don't actually delete the content, you just remove its entries in the Master File Table (MFT). The data is still on the disk and could be read using certain utilities. It's not efficient for BitLocker to encrypt free space on a drive, so BitLocker protects this empty space by creating a large placeholder file on the drive that uses up all space except for 6GB, to keep the system running during the encryption. The data, the placeholder file, and the empty 6GB are then all encrypted. (The efficiency hit for the 6GB is accepted in the interest of security). Once the encryption is complete, the placeholder file is deleted, ensuring any remnants of data on the drive have been replaced and the free disk space returns to the correct amount.

If you need the space back, you can suspend the BitLocker Drive Encryption then resume it once you no longer need the space. (Resuming will recreate the placeholder file.)