[Mi 3 General] SerjP:
But I'm still getting 'mount failed' during flashing via latest TWRP 2.8.7.0.

Installing '/sdcard/MK51.1-cancro-150627-RELEASE.zip'...
Checking for MD5 file...
Skipping MD5 check: no MD5 file found
Verifying zip signature...
I:read key e=3 hash=20
I:read key e=65537 hash=20
I:read key e=65537 hash=20
I:read key e=65537 hash=20
I:4 key(s) loaded from /res/keys
I:comment is 1738 bytes; signature 1720 bytes from end
I:whole-file signature verified against RSA key 0
Zip signature verified successfully.
I:Zip contains SELinux file_contexts file in its root. Extracting to /file_contexts
I:Legacy property environment initialized.
minzip: Extracted 5 file(s)
detected filesystem ext4 for /dev/block/platform/msm_sdcc.1/by-name/system
about to run program [/tmp/install/bin/backuptool.sh] with 2 args
grep: /system/build.prop: No such file or directory
Not backing up files from incompatible version: MK51
run_program: child exited with status 127
about to run program [/tmp/install/bin/otasigcheck.sh] with 1 args
Creating filesystem with parameters:
Size: 1337344000
Block size: 4096
Blocks per group: 32768
Inodes per group: 8176
Inode size: 256
Journal blocks: 5101
Label:
Transparent compression: none
Blocks: 326500
Block groups: 10
Reserved block group size: 79
Created filesystem with 11/81760 inodes and 10720/326500 blocks
Total files: 0
Total bytes: 0
warning: wipe_block_device: Wipe via secure discard failed, used discard instead

detected filesystem ext4 for /dev/block/platform/msm_sdcc.1/by-name/system
minzip: Extracted 1864 file(s)
created [/system/app/Bluetooth/lib]
created [/system/app/Bluetooth/lib/arm]
created [/system/app/BluetoothExt/lib]
created [/system/app/BluetoothExt/lib/arm]
created [/system/priv-app/DefaultContainerService/lib]
created [/system/priv-app/DefaultContainerService/lib/arm]
created [/system/app/Gallery2/lib]
created [/system/app/Gallery2/lib/arm]
created [/system/app/Camera2/lib]
created [/system/app/Camera2/lib/arm]
created [/system/app/PacProcessor/lib]
created [/system/app/PacProcessor/lib/arm]
created [/system/app/Terminal/lib]
created [/system/app/Terminal/lib/arm]
created [/system/app/NfcNci/lib]
created [/system/app/NfcNci/lib/arm]
created [/system/app/PrintSpooler/lib]
created [/system/app/PrintSpooler/lib/arm]
created [/system/app/FM2/lib]
created [/system/app/FM2/lib/arm]
created [/system/app/PicoTts/lib]
created [/system/app/PicoTts/lib/arm]
created [/system/app/webview/lib]
created [/system/app/webview/lib/arm]
about to run program [/tmp/install/bin/backuptool.sh] with 2 args
/tmp/install/bin/backuptool.sh: cd: line 102: can't cd to /tmp/addon.d/
md5sum: can't open '*sh': No such file or directory
find: /tmp/addon.d/: No such file or directory
find: /tmp/addon.d/: No such file or directory
find: /tmp/addon.d/: No such file or directory
cp: can't stat '/tmp/addon.d/*': No such file or directory
detected filesystem ext4 for /dev/block/platform/msm_sdcc.1/by-name/system
mount: failed to mount /dev/block/platform/msm_sdcc.1/by-name/system at /system: Device or resource busy
about to run program [/tmp/install/bin/makelinks.sh] with 1 args
script succeeded: result was [/system]I:Legacy property environment disabled.