All CRTs (and TVs) can emit X-Rays because of the high voltage electron beam used to cause the display to emit light. They aren't too powerful with modern CRTs as the voltages are lower than much earlier designs. (Growing up with 60's era color TVs, my parents were always admonishing me with "don't sit so close to the TV".)
There's no emission when it is powered off. However use caution working near the tube as it can store dangerous high voltage for quite awhile after power is removed (especially around the anode connector, which is plugged into the side of the glass tube). Use typical safety procedures when working around high voltages - remove jewelry/watches, and try to work using just one hand (that way, if you do get shocked, it won't go across your chest (bad, on the good/bad scale)).
Otherwise, there's no problem working on the hard drive. (Hope I didn't put you off of working on the Mac. I just wanted to make you aware of some potential safety concerns when working on electronics.)
