If your question is, does mounting an encrypted drive decrypt the entire drive, the answer is no. Mounting an encrypted drive unlocks the encryption key which is itself encrypted by the mount passphrase. It then passes the key to the driver so that blocks of the disk can be decrypted when read and encrypted when written. So, what is decrypted is in memory, not on the disk, and only for files in active use at the moment.
The vulnerability is a weak passphrase. If someone can seize an encrypted drive and guess or otherwise obtain the passphrase that unlocks the crypto key, then the entire drive can be decrypted just by mounting the drive and copying the files to an un-encrypted drive.
This vulnerability is why Ramona Fricosu is in jail. (Her husband ratted her out by supplying a list of possible passwords.)