Reformat old HDD.

I have several old laptop HDD's. I was looking to wipe them and use them as external storage. Is this feasible?

With my computer already on, I plugged it into SATA port 2. I turned my head for a moment and when I came back, my computer booted into the laptop drive. Is this to be expected?

What process should I go through to avoid that and to reformat, etc?