I found the thread listed above on adding a 2nd 2.5" SSD SATA hard drive to a new Precision 5510 laptop.
I have e a Dell 7480, is it possible to add a 2nd drive
No, it's not - the 7480 doesn't support any 2.5" drives. It supports only one M.2 2280 solid state drive - any other storage you need, must be an external device.
^ Correct, and actually even the Precision 5510 only supports another hard drive if it's equipped with the lower-capacity battery because the larger battery fills the space that's otherwise kept available for the spinning drive.