How to make TWRP read external SD card in Gionee M2?

[AC Question]

I've copied the custom ROM into the internal storage and mistakenly formatted the whole internal storage, data and OS. Now I couldn't install the ROM through external SD card since TWRP is unable to detect external SD cards. I'm even unable to change the recovery option. What to do now? How can I install a custom ROM into my Gionee M2??