Hey, I have the same problem and wish there was a little more attention on this problem. I recently cleared my hard drive cache on a different xbox. I didn't plug it into the core when I was done however. When I booted up my core system, a purple screen came up and said "updating". There was a bar that briefly started filling up. It also tried to sign me into xbl. This purple screen then cut off into a new screen saying. Update failed. "Couldn't find your Xbox 360 Hard Drive. If the hard drive is attached, contact Xbox Customer Support or go to www.xbox.com/support." The weird thing is, the hard drive wasn't attached at this time. I figured, oh I'm retarded, I forgot to attach the hard drive. Then I realized I was playing a 360 game, and the game should boot up anyways. I reattached the hard drive after rebooting and came up into the same purple screen. I disconnected the internet, rebooted the router and modem, went through troubleshooting steps, but nothing happened except for the same screen. The hard drive was fine, I tested it on the other 360 and it booted up and I could get into my profile and stuff. Finally I broke down and called support. After about 5 departments they patched me through to the xbl department and told me to connect the xbox to the modem without routers or switches. I did so and still got the purple screen. They said it was a problem with the xbox and that I had to send it in. I couldn't because I had already voided the warranty. (removed the seal) I can't access the dashboard or anything. It's just the purple screen that has one button available (b, back) If you press this button the purple screen refreshes and you get four more available buttons that pop up on the lower screen. You can't press y or a. If you press b or x they grey out and aren't available to press anymore. At that point your stuck in the purple screen. No dashboard, no game.
Someone please help. I'm thinking I might have to do a software hack or put linux in to boot up the 360.