OP wrote:
"1. How do I root from an external drive?
2. How do I select the internal drive?"
Before you can boot from an external drive, you need to have an external drive that is bootable.
You can create one by using either CarbonCopyCloner or SuperDuper to "clone" the contents of your internal drive to an external drive.
When booted, the external drive will look EXACTLY like the internal drive, because "it's a clone".
In fact, you may not even realize you're booted from the clone unless you check "about this Mac" (Apple menu) and see which drive is the boot drive.
Once you have an external drive that's bootable, you can do this:
1. Power down the computer, all the way off
2. Press the power-on button and IMMEDIATELY hold down the option key and KEEP HOLDING IT DOWN until the startup manager appears.
3. You should see all the bootable volumes in the startup manager window (includes internal and external drives and the recovery partition)
4. Use the pointer to select which drive you wish to boot from and then hit return.
5. The Mac should boot from the selected drive.