Hello
the temporary folder is created where you run the model, so the easiest way to proceed is to change directory and to run telemac in the folder you choose.
You don't have to copie all your files in that location, just you can give the path in the steering file like : FORTRAN FILE='C:/main_folder/sub_folder.../my_model'. In linux, you can use links which are a much more elegant way to do.
Otherwise you can run telemac with option --workdirectory which tells to telemac to run in the specified folder. I've never used this option before, but I guess it works well.
Finally, it is not recommanded to run telemac on an external hard drive since you will alterate computation effciency with more expensive I/O actions.
I hope that this helps
with my best regards
Riadh