Encrypt hard drive and keep the key in RAM. Could be recovered with a cold boot attack but that’s very advanced. The DOS virus ONEHALF would run as a daemon encrypring a block on the drive on each boot and intercept reads/writes to the encrypted part as if nothing ever happened. Only after encrypting ½ of the disk, it would reveal itself with an ominous

The decryption was eventually cracked by ESET and they developed a tool to recover the drive.