It is generally recommended to safely erase the content of a disk before encrypting it, to avoid some forensic attacks.
But is it necessary when that disk was encrypted before ? Wouldn't the data left be "random" since it's all encrypted ?
Information Security Stack Exchange is a question and answer site for information security professionals. It only takes a minute to sign up.Sign up to join this community
Random bits of data can make up fragments or even whole files. While that doesn't happen too often, it depends on the importance of the data and will-power of individuals wanting to gain access to it. If the HDD/SSD was not wiped completely, it could contain traces of previous files/fragments/encrypted data that can be obtained. Erasing the HDD completely will remove traces of any potential files and is not considered a bad practice.
Not an expert, but I would assume that partial/broken encryption is irrecoverable even with an encryption key.