I sense KirbyBoy wanted to link
http://www.360drives.com/howto or something like it (I do not have one for a slim right now).
Purchased means little- I have seen them sit on the shelves for years. Still if it was made around that time it is most likely a 7 series liteon but it might still be a benq. The link above has serials you can play with to figure out and you can also ID the main model easily enough (if it is a liteon it is certainly a 7 series and will need a probe or one of the full dump methods (go with a probe if you can)).
Back on topic so to speak there was a sticky
http://gbatemp.net/t123840-frequently-aske...360-and-backups but it is not updated as much as it could be and did not make the migration to this section. For all drives you get to dump the keys to the drive, build a new firmware and flash said new firmware to the drive. Building is just software but getting keys and getting it into flashing mode can and often does vary between drive models.
Flashing assuming you are not just using a standalone tool (the have just started to appear) takes some interesting commands on the part of the sata chipset which not all can do hence most guides directing you to various specific chipsets with via 6421 being the most reliable for this but a lot of onboard via and some later nvidia nforce stuff works just as well.
As mentioned JTAG hacks got blocked back in August 2009 (or just before if it was built or serviced in mid June 2009 or later)
Isos from an external. Afraid that is reserved for JTAG hacks and I guess the upcoming DVD replacement drive methods (if you know the wii think WODE).
Can you bypass the need for the disc to be in to play an installed game?
I have toyed with tokens in the past (unfinished discs and whatnot) but consider it a yes you need a disc to do it.
Compatibility is 100% for discs but currently 6 games have additional checks activated that cause troubles and presently do not work with the latest dash (cod modern warfare 2 and black ops, assassin's creed brotherhood, need for speed hot pursuit, fable 3 and Halo reach). This dash updates and counter flashing is a fairly new phenomenon (first hitting November 2010) but before that it was 100% give or take issues with the hacked firmware security being overzealous.
Undo- should you have the keys get a copy of the original firmware (downloads are not too hard to come by), wind the keys in using the same programs you used to make the hacked firmware and burn that.
High or low bricking chances?
As long as you have the keys and do not do something silly like force the power lead in the wrong way up you can pull it back (or buy a replacement drive) save for some hitachi drives which you might need to get a replacement for if you mess up badly.
"homebrew"
Not a patch on the original xbox* and probably not even up there with the wii (maybe slightly better than the PS3 depending on the person you are speaking to but I am not even sure about that any more). JTAG only too.
*before you ask I say get an original xbox and hack that instead as it saves a lot of hassle, you can do it with just a PC with an IDE port but failing that I do not mind sending a memory card/action replay and game to you- not that splinter cell runs much these days).
