<< Back "in the day", with the old desktop cases, we lived happily with hard drives mere inches from our tubes. I'd say if both devices are FCC class B certified you could tape them together and wouldn't see any problems...unless that hard drive contains an unshielded magnet somewhere... >>
Mine is FCC-B but I've noticed on same branded monitors if you put them next to each other (inches away) you'd get distortion.
That doesn't worry me so much as the possibility of losing data on the HD. I figure my desp space will allow me to keep them at least 6 inches apart. Enough?