Nexus 6 failed to update OTA 6.0.1 from MMB29V to MMB20X Urgent help please.
the april 2016 patch was downloaded on the air and I performed update but after restart, The bot screen came with "error" and then the red error message is shown directing me to g.CO/ABH in which flashing is suggested. note after restarting, many applications together with settings app crash. I performed the OTA update again and no chance. Is there any solution except flashing? if the answer is no, so how can I perform a full backup before flashing? My serious aim is no data loss. ftp works through wireless but too slow. the usb mass storage function shows an empty folder on PC. USB debugging is connected but adb shows no device. Also in bootloader mode adb shell failed. The device is not rooted and recovery is locked. Here's the brief last log:



Source: shamu:6.0.1/MMB29V/2554798:user/release-keys
Target: shamu:6.0.1/MMB29X/2704508:user/release-keys
..
performing verification
using existing stash /cache/recovery/.....
deleting recovery /cache/recovery/---/--
..bytes free on /cache (.. bytes needed)
failed to verify blocks (expected ..., read...)
stat "/cache/...." failed: No such file or directory
partition has unexpected contents
failed to read block for diff
failed to execute command [.....]
deleting stash 5a79....
script aborted: system partition has unexpected contents
E:Error in @/cache/recovery/block.map
(Status 7)

Installation aborted

Now tried to update from sdcard (stock rom downloaded for nexus) in recovery mode, got:
E:unknown volume for path [/sdcard]

-- Couldn't mount /sdcard.
Installation aborted

Repeated my try to update from ADB (adb sideload) but this error in the log:
sideload-host file size 1009749527 block size 65536
Finding update package..
I:Update location: /sideload/package.zip
Opening update package...
I: read key e=65537 hash=20
I: 1 key(s) loaded from /res/keys
Verifying update package...
E: footer is wrong
I:verify_file returned 1
E:signature verification failed
sideload_host finished
Installation aborted

I'm not sure even after data wipe the error remains or not, so prefer not to wipe yet, because the phone works!
Should I give up?
First off I'm assuming that the device is already flashing unlocked since you said you tried to flash the update from recovery. You shouldn't need a backup but make one anyways because recovery might be flashed over. Read everything first before you get started.
Download current image (the one you have now, you can try 29X or latest 30D), it's best to restore your system then update for guaranteed results and no misbehavior after reboot (I'm a little adventurous so I'd go for the 30D). unpack the .tgz and cd into the resulting directory.

fastboot flash radio radio.img
fastboot flash bootloader bootloader.img

Note the images aren't radio.img and bootloader.img, just do radio and tab and bootloader and tab.
Unpack the .zip that was extracted from the .tgz. cd into the resulting directory.

fastboot flash boot boot.img
fastboot flash system system.img
fastboot reboot

This should maintain all your data on the device after you reboot and I'm about 99 percent sure it'll work. So you shouldn't be needing to backup. If you run into problems after reboot you'll need to flash cache as well.
Reboot into bootloader again and use

fastboot flash cache cache.img
fastboot reboot

If it still doesn't work, you'll need to have a backup, reboot to fastboot and use the flash-all.sh that was extracted from the .tgz file. This would delete all your data but you can flash recovery again with fastboot flash recovery &lt;image filename&gt; and restore Data from the backup you did.
PS. If you have to flash cache you might lose some data but only relating to system apps and services.