"Error: -69842: Couldn't mount disk"
- Please do a complete backup before proceeding.
- You may check to see if your disk is unencrypted. Temporarily you could Turn Off File Vault from system preferences > security and privacy > File Vault > Turn Off
- Restart and login as administrator
- Open Terminal, and type "sudo diskutil verifyDisk disk0" and check for any boot error (type password at prompt)
- If any error gets detected, type "sudo diskutil repairdisk disk0" and press "y" (you must have total backup before this)
- If the Error: -69842: Couldn't mount disk continues to show up, try the command "mount /dev/disk0" (without quotes)
- If the Error: -69842: Couldn't mount disk still persists, you may try command "mount force /dev/disk0"
- If the disk gets mounted, or the repair is successful, type "restart" and hit return
- Try the HS installation after the disk is successfully repaired
# Clean HS installation is only possible if the partition map is successfully verified
# If the diskutil fails to repair and mount disk0, chances are your disk is partially or completely corrupted
# Best advise is to take the system to apple care, or reinstall OS X from scratch
## You may try in safe mode as mentioned earlier by others
## If you are confident, you may try fsck command or GParted to check and repair filesystem errors
## You may also tryto use Apple Hardware Test on your Mac - Apple Support
