Bumping this thread to share steps to solve an identical problem I faced. My Xiaomi Mi 3 W suddenly switched off one evening. It was rooted with TWRP. I had LiveBoot installed. On the initial boot, I could see log data with the phone unable to find/load resources and a "fatal error 6".
I booted into TWRP and found that it could not read any of the partitions. I sense the eMMC memory or the partition table somehow got corrupted. Everything was "0 Mb" in TWRP and wiping or formatting via TWRP was useless -- showing the "unable to mount storage" error.
The OS/data was not in any bootable state at this point.
After an hour of scouring many solutions on Google, the one that worked was: using the "fastboot erase userdata" , "fastboot erase system" & "fastboot erase cache" commands via the ADB shell on Windows.
After this, TWRP could detect my data partition. Cache and System were still 0 MB. But you can fix this by separately reformatting these partitions to "EXT4" via Wipe > Advanced Wipe > Repair/format > Format as EXT4. This was not possible earlier. The error message "unable to mount storage" would have vanished at this point. But you will see "failed to mount system" (invalid argument) for some reason. This isn't a problem.
You'll be able to flash/restore your ROM/backups etc again, now that TWRP detects all three partitions. Sadly, the old data is long gone in this situation. Would love to know what caused this whole issue in the first place.