Estou assumindo que essa criptografia é tratada pelo Windows BitLocker.
A Microsoft fornece um guia passo a passo.
- Click Start > Control Panel > Security > BitLocker Drive Encryption
- Find the volume on which you want BitLocker Drive Encryption turned off, and click Turn Off BitLocker Drive Encryption.
- From the "What level of decryption do you want" dialog box, click either Disable BitLocker Drive Encryption or Decrypt the volume.
Fonte: link