Ok, so a pretty big disk ~1-2TB (90% full, multiple partitions + 2 operating systems on it), needs to be encrypted and zeroed-out, and it's a bit too much data for backing it all up before encryption.
How do you recommend this to be done? Which tool should one use? How reliable would zeroing-out and encryption on-the-fly be anyway (don't want to loose any data in the process)?