What file path do i use for my addons if i play WoW with an external drive?
If the question is "Can I put my addons on a different harddrive" (for your personal preference or something) then No. You cannot.
If the question is "I installed WoW on an external drive, where do I put the addons ?" then find the folder for WoW on that harddrive, and put the folders with the addons in the WoW/Interface/Addons folder. Next time you log into WoW you will see a red button in the bottom left corner of the character selection screen that wasnt there before.