Is there a way to transfer files from a data DVD or CD to my Xbox HDD?

Yeah you can. Burn files as a DVD-ROM iso with Nero and it will read them fine. For the most part I know a modded Xbox will read UDF style isos as well, but I remember running into issues with UDF style isos using an older build of one of the dashes I was running.

When burning Xbox games it is always recommended to create an Xiso from game files rather than burning them as a UDF/ISO in Nero.

You can put a data CD/DVD in your Xbox and transfer them through a file browser. UnleashX has an excellent file browser, so does Avalaunch. XBMC has a file broswer as well. The disadvantage with this method is that the stock Xbox DVD-ROM drive reads data slow, 4x read speed max in most cases. So it will take a long time to transfer files for a full data DVD. FTP is preferred because it doesn't rely on discs. Even when transferring files from a data DVD, I did it from my PC's optical drive because it was faster and didn't tax the Xbox DVD drive's laser. Take note when trying to transfer files from a data CD, only a couple Xbox DVD-ROM drives will read CD-R's with any success. Here's a list of what Xbox DVD-ROM drives read what: Talk:Xbox Linux Boot CD/DVD Burning HOWTO - Xbox-Linux

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Thanks, bro.