This is probably late to the party (not going to read all 7 pages of posts) but here is the information in case it's useful at all.
I have lost my data 3 times (actually, it has never ever saved my data) on the xbox I originally played Castle Crashers on. However it is not the xbox I purchased the game on. (which is why I'm not posting this in the next thread "SAVE GLITCH without changing consoles (Help us help you!)"
I have two xboxes in my house. We'll call one "MINE" and one "KIDS". When I am not home, the kids play on "KIDS". When I am home, we'll either play online (one of my two kids on "KIDS", me on "MINE") or all of us hudled around "MINE" with the kids Live accounts on a memory card. In order to get my content to work properly, I buy ALL DLC content on "KIDS". Since the content is linked to the CONSOLE and the LIVEID, this links my content to the console "KIDS", but allows me to still play the game on "MINE" because I am logged in to live with my LIVEID. If I am unable to log in to LIVE, I cannot play with any of my DLC content, because it can't do the validation check. The content WILL play on "KIDS" regardless of their online status. Note that every piece of DLC content (20 or so arcade games, Rock Band Songs, Expansion Packs for games) work using this method, except for a small bug in "Ticket to Ride" which is fairly easy to get around, provided you know what to do.
So here begins the tale of how my data has never been saved. The first day we played, we all hudled around "MINE" and started playing. I made it to level 16 on the green knight. The kids didn't make it that high, but they were close. When we were done, I made sure we made it back to the map first, then exited the game through the pause menu.
The kids took their memory card upstairs, and played some more, both able to load their character. However when I tried to play solo, all my knights were at level 1 again (all animals lost, all weapons lost, etc), It was like I never played. However my "ranking" still showed level 16 on the "arcade details page". So I asked the kids to bring their memory card down, pop it in, and see what happens. They both try loading, and all their knights are at 1 again. I exit to the dashboard (so not to save over their memory card) and send them upstairs... their data loaded back on their machine (in fact they are upstairs just about to finish the game for the first time). So I have my son host a LIVE game from "KIDS". I join from "MINE". I am able to join his game, and we have some connection issues (dropping between levels, etc) but through that, my character remains leveled up (while I stay in the game).
I exit the game, and power down "MINE". Tonight I fired "MINE" back up to find all my characters are at level 1 again. Begrudgingly I ran through the first 3 levels, bought some potions, unlocked the barbarian, fought the barbarian boss with him, unlocked the achievement, exited to the map, exited the game (through the pause menu) and went back to the dashboard. Immediatly I fired the game back up, and presto... no unlocked characters, and everyone at level 1 again. Let me point out that my kids have not lost *ANY* data this entire time, using the "PURCHASED ON" xbox.
I don't know if this senario fits anyone else's issue with saving, but I hope it helps pinpoint the problem...
XBOX GT: DragonYen