Make old drive unbootable?

I just replaced my iMac's internal drive, and now I want to use the old one as an external storage place. So far so good; I could format it, put whatever I want on it, and be done. However, what I want on it is already there, in my Movies folder. Can I just delete all the other files and use it as an external drive, also on another mac, without issues? Or is there another step, like removing a boot flag or whatever? Or am I asking for trouble and should I just take the long route of erase and repopulate?

Typically, when I replace a hard drive, I'll clone the old to the new and then format the old and put it in a case. Sounds like you took the hard route and did a clean install on the new drive.

But barring that, what I would recommend is just copying the movies folder over to the internal, then erase the external and copy it back.