Hi Jerry,
Android tends to make this very difficult these days. The newer versions of the OS dont like applications writing anywhere other than a controlled place (which is what carcast defaults to.) The advantage is to protect you from bad applications. It also means when you remove carcast, all the datafiles are removed too. Generally speaking this is great for most users.
I also recommend you leave it as the default.
However, if you want to try anyway, go into Settings, then press on "Set data location"
I typically write down the current location and keep it so I can restore it. (you can just long press, and copy it)
then set the address to empty. Then return to the home screen. Carcast will show an error like this,
This shows the "base locations" that Android is telling carcast that is allowed to store files into.
If your phone shows another base path - presumably the SD card, then you can use that.
Let me know
Cheers
-bob