I wish I'd known about this a few months ago. I accidentally make a disk unbootable, but eventually, after many hours of struggle, I got it working again. This might have save me those many hours.
TestDisk - Partition Recovery and File Undelete
I found this reading the original Server Fault post on the huge data loss allegedly suffered by someone at a British web hosting company.
Linux command line mistake nukes web boss' biz • The Register
centos7 - Recovering from a rm -rf / - Server Fault