[Q] Flashing stock Android 5.0.2 image to Nexus 7 [2012], stuck at 'writing system' topic

So my Nexus 7 (2012, wifi only, 32 GB, Android 4.4.4, not rooted) is just over a year old, and I have had absolutely no problems with it until one fine day its battery was completely drained and even after charging it for over 3 hours, it refused to boot up. It was stuck at the 4 colored balls animation. I checked the bootloader and it said something like

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unable to mount /cache (invalid <something>)

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.
So I decided that since I will anyway have to do a complete wipe of the data on tablet, why not install Android 5.0.2 by getting the image from Google? But much to my chagrin, the command prompt has been stuck on

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writing 'system' . . .

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for almost an hour now. --Shouldn't the whole process be complete in 10 minutes tops?
Another thing is that adb does not detect my device, but fastboot does.
Also, the command,

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fastboot flash bootloader bootloader-grouper-4.23.img

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throws an error:

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writing bootloader . . .
FAILED (remote: (InvalidState))

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What could be the error? Is it an H/W issue? Or am I simply doing something wrong?
The commands I gave are:

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fastboot oem unlock
fastboot erase boot
fastboot erase cache
fastboot erase recovery
fastboot erase system
fastboot erase userdata
fastboot flash bootloader bootloader-grouper-4.23.img
fastboot -w update image-nakasi-lrx22g.zip

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