You are pretty going to have to reflash if you want to play anything newer than the scene release of deus ex.
AP2.5 - a method of protection revolving around extra checks to the disc. Only six normal games ever used it (Assassin's Creed: Brotherhood, Fable III, Need for Speed: Hot Pursuit, Halo Reach, Call of Duty: Modern Warfare 2 and Call of Duty: Black Ops) but XGD3 discs (covered below) all use it. MS initially updated it causing people to reburn with each dashboard update but eventually they did a silent update making every console have unique checks. It has since been put to rest though with the release of LT+3.0 so you just need to burn a copy of the game with the new patches and it should work without hassle.
Alongside this we also saw XGD3 which is a new disc format MS cooked up to allow more space but it also means you need a hacked liteon iHAS drive to burn it with (they are fairly common, good quality and about the same price as most other DVD burners). To support this new format MS reflashed all the drives with a new firmware which was then hacked but only with the LT line of firmwares and around the time deus ex released we saw official games carry the update that reflashed drives (all new games will now) and that also required the games they were on to have that dash (no wave patching to dodge it).
Fortunately as you say you are on 1.6 you should be able to get your keys back if you do not have the somewhere else (LT is considerably more annoying to grab keys from).
I should also mention that older games that do not have splitvid (many will and most released since LT will) will need to be reburned with splitvid- ABGX360 can add it easily although you might want to also download the wave patches as well to put the waves back to where they should be.
Also if you are rocking a banned 360 you might consider adding in the reset glitch hack as it brings it into line with JTAG systems (region free, games from hard drives both USB and onboard, XBLA and DLC just by adding it, homebrew and more) for about $30 at most.