Damaged Save Game
I got Assassin's Creed 2 the day it came out, took my time doing the missions, collecting feathers/chests, doing side missions and exploring the city. Anyways, 4 days ago, I had just gotten to Sequence 14 and the game was autosaving, when all of a sudden, my Xbox froze.
Now I didn't really think much of it at the time, after all, what Xbox doesn't freeze once in a while? I turned my Xbox off and went on the internet to look up some Youtube videos. 30 minutes later, I was back on my Xbox and ready to play once more. All of a sudden, I was hit by a wave of pure terror: 
Damaged Save Game
I lost about 20 hours of gameplay for no apparent reason. As this happened to anyone else? Is there any possible way of correcting this error? If not, is there any way to turn autosave off in case anything like this ever happens again?
This has never happened to me for any other game that I've ever played since I bought my Xbox about 3 years ago, and I'm utterly disgusted from having my progress in the game literally erased in the split second that it took for my Xbox to freeze completely.
Unfortunately, this has indeed happened to some people. Both on the 360 and PS3 versions.
The plus side of the 360 version is that it is pretty easy to make backups of your saves if you have the right hardware. Our notorious webmaster stabguy has a video on his YouTube channel showing what you need and how to do it. It is entitled "Skipping Repetitive Parts of Assassin's Creed" and it is very informative. It works on AC2 in the very same way that he demonstrates it for AC1.
Unfortunately, that won't help your existing situation. However, it may be possible to repair the save using the same hardware from stab's video plus some helpful homebrew software. If you acquire the necessary hardware, send me a private message and I'll fill you in on a process which I'm confident will fix the save. I can't make any promises though. 
Unfortunately, I completely erased the corrupted file.
It's a shame that his has been happening to so many people. Hopefully, some sort of update will be released to resolve this issue. It's so frustrating to lose everything like that.
I'll replay the game again in a couple of days (I went back to the first game to look for the rest of the flags/Templars that I'm missing). I was only missing one Codex page and a couple of Glyphs and I was waiting until I had collected everything to read the Codex pages,watch The Truth video and beat the last level...
Anyways, do you think that purchasing a memory card would help? As long as I save directly to my hard drive and copy the data over to a memory card after every gameplay, even if my hard drive's data becomes corrupted again, I'd be able to put my clean save game from my memory card back on my hard drive, correct?
Or would simply copying my save game on my hard drive work? When a game is autosaved, does it save to both copies, or only to the original?
Thanks for the help, I just want to know as much as I can about this so it doesn't happen again. 
You can't have duplicates of the same save on a hard drive, but you can certainly copy your save to a memory card any time you like.
I've been screwed over by corrupted saves multiple times over the years. I'm a little paranoid now, so I always make occasional backups whenever I play any lengthy game. I stopped and backed up my AC2 save to memory card (and PC) several times during my first playthrough.
Since then, I have gone through the game again, backing up the save before every single story mission. However, this is for ease of making maps and videos, not due to paranoia. 
I see. Well, thanks a lot for the help. I really didn't want to risk replaying it just to lose everything again.
A memory card is definitely going on my Christmas list, now 
It is entitled "Skipping Repetitive Parts of Assassin's Creed" and it is very informative. It works on AC2 in the very same way that he demonstrates it for AC1.
I actually renamed it today to reflect the fact that it also works on AC2. It's here on The Hidden Blade: Replay AC1 and AC2 Story Missions on Xbox 360
Do you think that purchasing a memory card would help?
Yes. A memory unit gives you a single savegame backup (see video above). If you want multiple save points, the Transfer Kit is a more affordable solution than buying additional memory units.