Your question could be improved with some example screenshots of adverts you've seen, otherwise it could really be anything.
I think it's most likely that in the pay-per-word cramped space of a magazine trading advert, the phrase "with X-Drive" means the item comes with an external (or extra) floppy drive, which was a common Amiga expansion for all models for productivity use.
Image CC BY SA, author: Multicherry
Similarly "with 2x Drive" could mean it comes with 'two additional drives' (and very unlikely but still possible that it would come with a 2X speed CD-ROM drive).
It's also possible you were seeing adverts for Amigas with the animation package X-DVE (sometimes called X Drive), which was given away with Amiga Format issue 88.