According to this link:
"Encrypted Hard Drive Architecture
Encrypted Hard Drives utilize two encryption keys on the device to control the locking and unlocking of data on the drive. These are the Data Encryption Key (DEK) and the Authentication Key (AK).
The Data Encryption Key is the key used to encrypt all of the data on the drive. The drive generates the DEK and it never leaves the device. It is stored in an encrypted format at a random location on the drive. If the DEK is changed or erased, data encrypted using the DEK is irrecoverable. "
It sounds to me like there is in fact an important file which is encrypted and stored randomly on the hard drive; probably in the "Windows" folder or "Program Files" folder. The odds of finding this file seems low and the risk of corrupting the drive by deleting these folders seems high.
The best solution that I am seeing at the moment is to keep these folders on the hard drive and forgo using that storage space for another purpose.
Alternatively, I could upgrade to Windows 10 Pro (which I don't have - I have Windows 10 Home), decrypt the hard drive and then delete the "Windows" folder and "Program Files" folder.
I can decrypt the device in Windows 10 Pro by:
1) connect the device and unlock it
2) open an elevated command prompt (rightclick cmd.exe and select "run as administrator") and launch the command
3) Launch the command
manage-bde -off d:
(substitute d: for the correct drive letter)
These steps can be found here:
All things considered, until I stumble upon a friend that has Windows 10 Pro (I have Windows 10 Home) and do the above steps, I will not delete the "Windows" or "Program Files" folders.