You mean encryption can prevent a malware infection? If it can then there's no need of AV/AM software, right?
Also, if the malware screws up the encryption (although not being able to break the password-protected drive) then I think you'll not be able to decrypt the drive.
I would like to see you drop a malicious payload on a hard drive that you cannot even access. If you can, please do show me, because I'm really curious.