Bungie has announced it is aware of this issue and it is currently investigating the cause and solution to the problem. Also, the developer announced that Bungie PRO subscriptions are currently offline due to a small issue that is also being investigated.
Gamers who played Halo 3 on their console and decided to finish the fight in a friend's Xbox 360 got a nasty surprise. There is a bug that stops you from transfer campaign progress to another Xbox 360 console whether you use a memory unit or take your hard drive. You can load up any mission or insertion point you've previously completed but you won't be able to "resume" from your last saved spot. You simply get a "Unsafe to Respawn" message and then you're taken to the menu screen.