Xboxes can be fussy beasts with some drives reading some media and others reading other. I've personally not found any parity between drives from the same manufacturer. A friend bought his xbox at the same time as me - same drive, probably from the same xbox production batch etc... - but his will read DVD- media just fine.
It doesn't really matter though. As long as it reads one type of recordable DVD media then I'm not bothered!!!
@ Synchro - My offer of duplicating DVD's still stands. Have myself a couple of nice Pioneer DVR109 16x writers now along with a DVR106!