This is known as a F.R.A.G. (F
reen Lights) aka "Christmas lights".
There are a lot of causes of the problem.
Anything from the bios missing to bad RAM or other failures. Unfortunately this type of failure will generally require the replacement or repair of the motherboard. Remember that the Motherboard and HDD are paired, only the motherboard knows the password to unlock the drive so just swapping out the motherboard is not an answer.
More about this ---> http://www.llamma.com/xbox/Repairs/system_failure.htm
A lot of times this has to do with the motherboard or the modchip --- for example, if you botch a chip installation, you will likely get the christmas lights. Also, my chip has a 1MB capacity for bios which can be split over 4 (256kb), 2 (512kb), or 1(1mb) bank(s). I believe 2 out of the 4 banks that I'm using contain bad bios, and when I set my switch to load from those banks I get a FRAG.
So, if your chip has multiple banks, try switching to a different bank. If you had a bad flash, you will need to reprogram your chip one way or another.