Should I Install PT9 on an External Harddrive?

Newbie here. I've loaded and recorded a few songs on PT9 on my Macbook Pro. I'm not 100% sure, but I suspect PT9 has made my Macbook unstable. It freezes occasionally now, and it's never done that before. I suppose my Mac could have picked up a virus, but the timing of PT9 install corresponds precisely to the Mac beginning to freeze up. I recall a poster here suggesting that PT9 be installed on an external harddrive, and I'm wondering if it's to make everything run smoother? I'd appreciate comments on the best location for a PT9 install, and why an external HD is the way to go. I'd also appreciate any recommendations for a good external HD for a PT9 install. Many thanks.

The general recommendation is to install the Protools application on the system drive. It is the session files that should be placed on a separate drive (not the application).

Ahh. Thank you. So, leave PT9 installed on my Mac's hard drive, and lay down the tracks on the external drive? It must not be PT9 that's causing my Mac to freeze then, since it will freeze even when I don't have a recording session open. Unless the existence of unopened sessions could be causing the freeze- but that seems unlikely to me.