Hi Marc,
the revision-log says, starting with BRouter 1.5.2, "easy install on external SD (but RD5s will be deleted on uninstall)"
So what you want to do should be the default.
On initial install, the base-dir-selection dialog should show he 2 special directories "../Android/data/btools.routingapp/files" for the internal and the external SD, together with the free spaces, so you should recognize which one is the external.
In order to get to the base-dir-selection dialog again, you must uninstall the BRouter app first.
But of course, the Android universe is too diverse to kno for sure what will happen on different devices. So maybe just tell me what the base-dir-selection dialog looks like?
regards, Arndt