Okay, here are my problems...
1. I can load up from my boot disc fine (slayer's auto-installer). Turn on and off a few times, and it boots to the setup screen. When I do the full format and install on my retail HD, I take out the disc, and start up my Xbox, and get error 16. Any help with that?

2. When I try to boot up with my upgrade hard drive (maxtor diamondmax 16 300gb) I can't get Evox to boot for anything.

Now, when I start my xbox with nothing in the DVD tray, I get the Xbox dash opening screen, with the little Evo X icon in the top of the screen, then I get one of those numbered errors. I really need some help. I've been at this for 2 weeks now... Oh yeah, and my bios is the M8 bios.

In scenario 2, what error do you get?
It sounds like BOTH of these HDDs may actually be dead. Even though it says you can format and install to your retail disk, it is likely failing: I had a puzzling problem (at the time) happen to me. EXACTLY the thing you described in 1. and it turned out replacing the HDD fixed it.
In Scenario 2, have you thought about reinstalling Evox to the HDD (just Evox, not formatting or anything)? Have you tried that, or have you done that and it still gives you the error message?

I get error 7 on the upgrade, and error 16 on the retail. It's weird, because my computer can access both drives (Can't format them, though). Maybe I'm not using the right jumper settings?

Is there no one else that has any help? I'm really desperate here... (and broke )