[Q] Unbrick Transformer topic

Hello at all!
In the beginning I want to tell you that I'm no native speeker, so please excuse me when my language is a little bit strange ;)
I'm here to ask you for help to unbrick my Aszs TF300. I have the following problems:
a) I can not mount internal sd card, but I can mount my external sd card as /system
b) everytimes when I start my tablet (just holding the power button) it boots into recovery (TWRP 2.6.3)
c) adb and fastboot don't see my Tablet

I searched for the mistake and found out, that my Tablet is bricked. But it is still possible to move files from the external SD card to the SD card, but they are lost when I reboot. Additionally I can use an Android terminal command.

Is there any way how to repair it (for free)? I had the following idea: It is possible to tell the Transformer to use the external SD card as internal SD card? Maybe I can install a new Android version on the external card and so I'm able to boot? Are there any other ideas?
Currently the tablet is dead... so I'm open for ANY idea and I would be very happy if someone could give me a tip. :cowboy:

Thank you very much for reading! :good:
best regards
Trolli1991