El-capitan crashing while booting
I/O Error usually means your disk has hardware failure. I would backup everything ASAP and change the disk. Disk Warrior will probably not solve the issue as it's only repairing a corrupted filesystem.

If you can mount it at some point (try repeatedly), use Carbon Copy Cloner to backup your important data on a new drive. If you can't, try some recovery software like Data Rescue.