It is possible to keep your Catalog,images,Plugins on an external drive.
I would recommend you to store your Presets with the Lightroom catalog itself.Its been there purposely to allow you to carry presets, templates etc with you when your catalogue resides on an external hard drive and you frequently attach it to other computer.
This will also help you to locate them easily in External harddrive.
Thank you. Do you have to manually track down all of your presets & templates? When I selected keep presets with catalog it just show default templates.
This is only for third party presets you save
When you check this option It creates a Folder called 'Lightroom Settings' which is stored in the same folder as the catalog that is currently open.
Lightroom will write any new user presets that the user creates into the appropriate sub-folder within the Lightroom Settings folder (e.g. a new develop preset will be stored in [Catalog Folder]>Lightroom Settings>Develop Presets>User Presets).
Do you have to manually track down all of your presets & templates
Yes ,You could see all your presets in Lightroom from Catalog Setings>>Show Lr Ppresets Folder
By default the location is:-
/Users/[user name]/Library/Application Support/Adobe/Lightroom/[preset or template folder]/[preset or template filename.lrtemplate (MAC)
C:\Users\[user name]\AppData\Roaming\Adobe\Lightroom\[preset or template folder]\[preset or template filename.lrtemplate] (Windows)