I don't recommend moving your project files to your internal harddrive. I would leave them on the external drive, as long as its a fast drive.
I have had the same problem with Motion 5 freezing after I hit Play, but it only happens when I immediately hit Play after making a change. I know it might sound silly, but give it a good pause before hitting Play. This has worked for me.
Also, have you unchecked some of the playback options in the upper right corner? Try turning off lights, shadows, depth of field, etc. unless you absolutely need to have them on to effectively work on a certain area of your project. Those settings can and will drastically reduce playback, and understandably so. You can always turn back on any of those settings before final export. Or how about a RAM preview? I agree that always having to RAM preview to see anything in After Effects is annoying, but sometimes --not always-- this is necessary with Motion 5 too.
Another thing you can try is to truncate layers once the object has left the screen. Even though you cannot see an object once it's no longer "in" the canvas, if the layer still exists in the timeline Motion is still going to process all of it, which puts an unnecessary strain on your system. For example (though I know you don't have particle emitters in this particular project), setting particles' life-cycle so that they die off once they leave the viewable Canvas area is a good idea, because Motion will still process them otherwise, and this is a waste of resources.
Let me know if you have any luck.