first off, don't worry, you didn't delete the firmware. you probably accidentally deleted some files that you shouldn't have. The good news is that xbox hard drives are keyed to the motherboard and you more than likely haven't removed the "locked" state that the hard drive is set to. So, what to do now to fix this? well, you will be happy to know that this is surprisingly easy and won't require a soldering iron at all.
1. put the xbox back together with the top off.
2. boot it up to the error message or just before that. you will actually hear (in most hard drives) a click sound when the hard drive lock is disabled. It *has* to be disabled so that the motherboard can read it and then it fails to read it. we're trying to catch it at that point. Most Xboxes (1.0-1.5 i believe) will leave the hard drive unlocked on reading failure. 1.6 typically has for me as well.
3. turn on your computer and let it boot to XBOXHDM's 4 choices
4. complete the hot swap
5. go into option 1 and then use whatever hot swap method you were going to use in the first place, probably NDURE soft mod.
6. once completed shut down PC, disconnect IDE line from Xbox, power off the xbox, plug your hard drive back in and boot it up again.
in a worst case scenario, you should be able to put stock C and E onto the XBOXHDM disk and just wipe the whole hard drive and put the stock xbox files back on it. if you have access to an already modded xbox, you could try (I've never done it but others say it works) putting them next to each other, pulling the hard drive once unlocked (leaving power on of course) and plugging it into the IDE on the modded xbox and then booting the modded xbox with an AID or Hexen or Slayers disk and do the soft mod on it from there.. then you can power off the chipped box, power off the non chipped box and put it back in the original xbox and reconnect everything and it should power on fine..
The previous poster is right with how to fix via chip, but the chips are so painfully hard to find and are more expensive than the actual box that it really isn't worth it..