If you put MP3 ring tone in the Mini SD card of a Asus P535, you will know there will be some issues.
When you connect your mobile phone to the PC, it then becomes a card reader. And the Mini SD becomes accessible only through the USB port, but not to the mobile phone itself. That means you cannot see the Mini SD card, you cannot access the file through any installed applications, etc.
Another important implication is that, the MP3 ring tone which is stored in your Mini SD will not be accessible, that means when there is incoming call, the ring tone cannot be used and you will fall back to the default ring tone. That could be confusing enough. You might not even recognized that it is your phone which is ringing! Since the P535 recharges via USB, you will even lost the ring tone during charging! That is troublesome enough if you use your MP3 files for alarm!
To solve the problem, one of the easiest way is to put your MP3 ring tone in your phone instead of to your Mini SD. Of course, it is not a very smart idea since MP3 ring tone could be quite large which wastes valuable memory spaces.
A better way is to set the USB to ActiveSync mode, and your Mini SD will remain accessible by the mobile phone, and thus the ring tone will still be accessible.