[Nexus 6 Q&A, Help & Troubleshooting] vigkumar:
Thanks a lot for your assistance on this. I followed your exact instructions and it is given below.

C:\Users\Vignesh\Downloads\Nexus\platform-tools-latest-windows\Latest\platform-tools>dir *.img
Volume in drive C is Windows8_OS
Volume Serial Number is 40E0-62B9

Directory of C:\Users\Vignesh\Downloads\Nexus\platform-tools-latest-windows\Latest\platform-tools

01-01-2009 00:00 8,686,822 boot.img
01-01-2009 00:00 6,398,184 cache.img
26-09-2017 16:53 118,279,680 radio.img
01-01-2009 00:00 14,206,186 recovery.img
01-01-2009 00:00 1,619,795,140 system.img
01-01-2009 00:00 142,201,816 userdata.img

C:\Users\Vignesh\Downloads\Nexus\platform-tools-latest-windows\Latest\platform-tools>fastboot devices
ZX1G525DZX fastboot

You're right. Initially it was DEVICE is UNLOCKED. Status Code 3.

C:\Users\Vignesh\Downloads\Nexus\platform-tools-latest-windows\Latest\platform-tools>fastboot reboot-bootloader
(bootloader) slot-count: not found
(bootloader) slot-suffixes: not found
(bootloader) slot-suffixes: not found
rebooting into bootloader...
OKAY [ 0.085s]
finished. total time: 0.085s

Once reboots, it says DEVICE IS LOCKED. Status Code 2. I had to long press the power key, which unlocks the DEVICE.

C:\Users\Vignesh\Downloads\Nexus\platform-tools-latest-windows\Latest\platform-tools>fastboot flash boot boot.img
(bootloader) slot-count: not found
(bootloader) slot-suffixes: not found
(bootloader) slot-suffixes: not found
(bootloader) has-slot:boot: not found
target reported max download size of 536870912 bytes
sending 'boot' (8483 KB)...
OKAY [ 0.285s]
writing 'boot'...
(bootloader) Failed to erase partition
(bootloader) Failed to flash partition boot
FAILED (remote failure)
finished. total time: 0.338s

C:\Users\Vignesh\Downloads\Nexus\platform-tools-latest-windows\Latest\platform-tools>fastboot flash radio radio.img
(bootloader) slot-count: not found
(bootloader) slot-suffixes: not found
(bootloader) slot-suffixes: not found
(bootloader) has-slot:radio: not found
target reported max download size of 536870912 bytes
sending 'radio' (115507 KB)...
OKAY [ 3.737s]
writing 'radio'...
(bootloader) flashing modem ...
(bootloader) Failed to erase partition
(bootloader) Failed to flash modem
FAILED (remote failure)
finished. total time: 3.774s

C:\Users\Vignesh\Downloads\Nexus\platform-tools-latest-windows\Latest\platform-tools>fastboot flash recovery recovery.img
(bootloader) slot-count: not found
(bootloader) slot-suffixes: not found
(bootloader) slot-suffixes: not found
(bootloader) has-slot:recovery: not found
target reported max download size of 536870912 bytes
sending 'recovery' (13873 KB)...
OKAY [ 0.580s]
writing 'recovery'...
(bootloader) Failed to erase partition
(bootloader) Failed to flash partition recovery
FAILED (remote failure)
finished. total time: 0.617s

C:\Users\Vignesh\Downloads\Nexus\platform-tools-latest-windows\Latest\platform-tools>fastboot flash system system.img
(bootloader) slot-count: not found
(bootloader) slot-suffixes: not found
(bootloader) slot-suffixes: not found
(bootloader) has-slot:system: not found
target reported max download size of 536870912 bytes
sending sparse 'system' 1/4 (522524 KB)...
OKAY [ 18.848s]
writing 'system' 1/4...
(bootloader) Failed to erase partition
FAILED (remote failure)
finished. total time: 18.901s

C:\Users\Vignesh\Downloads\Nexus\platform-tools-latest-windows\Latest\platform-tools>fastboot flash userdata userdata.img
(bootloader) slot-count: not found
(bootloader) slot-suffixes: not found
(bootloader) slot-suffixes: not found
(bootloader) has-slot:userdata: not found
target reported max download size of 536870912 bytes
erasing 'userdata'...
(bootloader) Failed to erase partition
FAILED (remote failure)
finished. total time: 0.037s

C:\Users\Vignesh\Downloads\Nexus\platform-tools-latest-windows\Latest\platform-tools>fastboot flash cache cache.img
(bootloader) slot-count: not found
(bootloader) slot-suffixes: not found
(bootloader) slot-suffixes: not found
(bootloader) has-slot:cache: not found
target reported max download size of 536870912 bytes
erasing 'cache'...
(bootloader) Failed to erase partition
FAILED (remote failure)
finished. total time: 0.016s