Take a look over this
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You cannot just use any HDD, it has to be a Western Digital. Also, you can only use 20/60/120 GB partitions.
Basically what you need to do is get a hold of the correct HDD, format it in Fatx, and put a valid HDDSS.bin (20/60/120 gb versions. If you put a 20gb your xbox will only use 20 gb of the hdd, 60, 120 etc...)