“An unexpected error has occurred during recovery” was the error message I got after trying to recover my Acer C740 Chromebook with multiple different USB sticks flashed with the Chrome Recovery tools by two different computers. The recovery process would always start by the verification of the USB stick and then end in the stated error right when the verification ends.
Reason: the computer’s (Chromebook’s) SSD had died and wasn’t recognizable.
The bad: the Chrome OS recovery process, boot process, or whatever process, does not tell you that the SSD has failed and isn’t available. It will just fail and provide you with a generic error.
The solution: when you boot up without the recovery USB stick, and you’re welcomed by the error screen (“Chrome OS is damaged and needs to be repaired” etc.), press SHIFT. Shift displays a boot process log on the screen and that tells you if the Chromebook cannot interact with the SSD.
It would be quite nice to have the official recovery tool to provide you with this information rather than giving the error message from this post’s first line.