I also have few of the other original Xbox DLC such as Halo 2 map packs stored on this HDD. If I can't transfer the DLC data, I would rather just buy another wireless network adopter and another fat 360 HDD and switch between HDDs than a new slim Xbox 360...
I'm pretty sure that the cable won't work. That cable will transfer your saved games over properly, but I don't think it'll work for DLC, as the licences for DLC are tied to each console. Even if it transfers the content, you won't be able to relicence it online anymore with the original Xbox support gone. Unless the cable itself somehow updates the licences, but I'd seriously doubt that.
My 360 just RRoD'ed a month ago, and I got a refurbished one back. I can't use Yavin Station or Ninja Gaiden's DLC anymore because I have no way to renew the licences.
So you might not want to toss away your old console yet.
No, you can't go online with them like with the 360 games because it signs out your profile / gamertag upon starting the game, I believe. And even if you could, they disabled the servers for the original games a few months ago anyway. This wouldn't have been an issue in March.
Anyway, if I'm wrong, I'd love to know, because I now have a lot of useless DLC sitting on my hard drive that I want to play again.
Files can be transferred as original Xbox save files. The transferred original Xbox files will then remain as "Unknown" files until you boot the Xbox game associated with it in the new machine/HDD. Save files seem to recover just fine, however the DLC files are identified as corrupted files and will not work on the new Xbox 360 HDD.