[Nexus 6 Q&A, Help & Troubleshooting] Symnonymous:
Phone was on charger overnight, went to use it and just boot loops. Was fine the day before. so I tried to flash a rom to no prevail, so I loaded NRT to flash 5.1 only to keep getting Failed bootloader and radio install. I can get to bootloader and recovery, but data, cache and storage cannot be mounted. the following are my NRT results:

Flash Stock + Unroot...

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target reported max download size of 536870912 bytes

sending 'bootloader' (3807 KB)...

OKAY [ 0.125s]

writing 'bootloader'...

(bootloader) flashing partition ...

(bootloader) This may take a few seconds if a

(bootloader) different partition table is being

(bootloader) flashed since we need to backup

(bootloader) and restore a few partitions

(bootloader) Flashing primary GPT image...

(bootloader) Flashing backup GPT image...

(bootloader) Failed to write EFI backup GPT header.

(bootloader) Failed to flash partition

FAILED (remote failure)

finished. total time: 0.398s

rebooting into bootloader...

OKAY [ 0.000s]

finished. total time: 0.016s

target reported max download size of 536870912 bytes

sending 'radio' (85422 KB)...

OKAY [ 2.750s]

writing 'radio'...

(bootloader) flashing modem ...

(bootloader) flashing mdm1m9kefs1 ...

(bootloader) flashing mdm1m9kefs2 ...

(bootloader) flashing mdm1m9kefs3 ...

(bootloader) flashing versions ...

(bootloader) Failed to flash partition versions

(bootloader) Failed to flash versions

FAILED (remote failure)

finished. total time: 3.320s

rebooting into bootloader...

OKAY [ 0.000s]

finished. total time: 0.016s

target reported max download size of 536870912 bytes

sending 'boot' (7681 KB)...

OKAY [ 0.266s]

writing 'boot'...

OKAY [ 0.047s]

finished. total time: 0.313s

rebooting into bootloader...

OKAY [ 0.000s]

finished. total time: 0.016s

target reported max download size of 536870912 bytes

erasing 'cache'...

OKAY [ 0.016s]

sending 'cache' (6248 KB)...

OKAY [ 0.203s]

writing 'cache'...

OKAY [ 0.047s]

finished. total time: 0.281s

rebooting into bootloader...

OKAY [ 0.000s]

finished. total time: 0.000s

target reported max download size of 536870912 bytes

sending 'recovery' (8273 KB)...

OKAY [ 0.286s]

writing 'recovery'...

OKAY [ 0.063s]

finished. total time: 0.348s

rebooting into bootloader...

OKAY [ 0.000s]

finished. total time: 0.016s

target reported max download size of 536870912 bytes

sending sparse 'system' (522112 KB)...

OKAY [ 18.386s]

writing 'system'...

OKAY [ 3.252s]

sending sparse 'system' (510197 KB)...

OKAY [ 18.144s]

writing 'system'...

OKAY [ 3.211s]

sending sparse 'system' (515601 KB)...

OKAY [ 18.700s]

writing 'system'...

OKAY [ 3.356s]

sending sparse 'system' (188485 KB)...

OKAY [ 6.640s]

writing 'system'...

OKAY [ 1.172s]

finished. total time: 72.878s

rebooting into bootloader...

OKAY [ -0.000s]

finished. total time: 0.016s

target reported max download size of 536870912 bytes

erasing 'userdata'...

OKAY [ 0.016s]

sending 'userdata' (138868 KB)...

OKAY [ 4.466s]

writing 'userdata'...

OKAY [ 0.896s]

finished. total time: 5.378s

rebooting...

finished. total time: -0.000s

Booting up your freshly flashed stock device...

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Wait for your device to finish booting up...

- It may appear to be boot looping; just wait...

- It could take 5-10 minutes; please be patient...

When its finally booted back up, please remember

to re-enable USB debugging if you plan on using

the toolkit to perform other operations.

Press any key to exit...

Any Help?