I'm not too familiar with CrypoSuite or is that supposed to be CryptoSuite? Anyway, a few years ago having a file on a seperate drive would have protected you from most viruses, but the faster and better bread of this day and age can jump across the drives, just fine. the 'nasty' you speak of, would likely corrupt or delete the file at it's binary or base code, thus it wouldn't matter that the file is encrypted.
Encryption is simply to protect against unauthorized access, if you were able to encrypt or protect the entire drive with CryptoSuite, then it would likely protect your image, unless the 'nasty' was specifically designed for such a software.
The only real protection is to burn your image so it is read-only!!