New answers tagged bootloader
It looks like you've got the wrong factory image. You're trying to flash the bootloader for flounder (which I believe is for the Nexus 9). You need to download a Nexus 5 factory image which should contain a bootloader called bootloader-hammerhead-hhz12d.img (or similar)
I now downloaded the hammerhead-lrx22c-factory-0f9eda1b instead of the volantisg-lrx22c-factory-a9668749 and it worked flawlessly.
I was seeing a very similar issue but in my case the "Formatting ..." step was not followed by a failed to mount message. In my opinion there is a hardware issue with your unit -- if the format fails such that the resulting partition can not be mounted, there's not much that can be done except attempt a warranty repair.
You don't have an unlocked bootloader on your s5. I know this, because to date, there is no known unlock method for the bootloader. To restore to stock, uninstall safestrap (if applicable) then use Odin or Heimdal to flash a factory image.
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