From majornelson.com:

When Preview Program members start receiving the Xbox 360 system update next week, one of the changes is that unauthorized Memory Units will no longer work with the Xbox 360. If you've moved your profile or saved games onto one to "back it up," you'd better move it back onto an authorized Xbox 360 storage device prior to taking the update. If you continue to use an unauthorized Memory Unit after the update, you will not be able to access your stored profile or saved games.

If you need help identifying officially licensed Xbox 360 storage devices or accessories you can read more about the licensed accessories program on Xbox.com
It's unsure if they mean devices like Datel's MAX Memory or homemade Xbox 360 HDDs, but I guess we can assume they will try to block both. There's no 'stealth' what-so-ever with home-made 360 HDDs so it would be pretty easy for MS to detect them. Back to paying 150usd for a 40usd 120gb 2.5" SATA drive with a fancy case?