Making Tasker write into the external SD card
Have a look at https://groups.google.com/forum/#!topic/tasker/zuY21R_bvyU
where Pent said:

Here's a nice link explaining the situation:
https://groups.google.com/forum/#!topic/eyes-free/rspqjlmu_ss

As I understand it, if on lollipop or 'portable' mode on marshmallow are used, access is required via a special API. I've looked at this API (DocumentFile) and it's going to require rewriting huge chunks of code used for the file functions, plus some awkwardness with the user pre-accepting file operations
on external SD (and I don't know how long that preacceptance stays valid, potentially making it
useless for later automation)

Summary, I'm not going to be doing this in the near future, if more mainstream devices start to get SD I may reconsider.