I'm having the following problem.
When I try to boot my MacBook Pro 13" mid 2009 with "El Capitan" installed from my internal HD, a prohibition sign appears on the screen and eventually my mac turns off. I have an external HD with El Capitan installed and it boots nicely. When I try to repair the volume on Disk Utility I get the following message
I can use the terminal to force unmount the volume and then repair it. But after that, when trying to repair it again I get the same mistake, and also the HD still won't boot. I have discarded a defective SATA cable and a corrupted HD since I'm able to copy files from the HD to my bootable HD without problem, so I'm guessing this issue is software related.
Any help would be most welcome.
Update: When I use first aid on the main drive I get the following message:
Even after doing that, the partition won't boot or unmount for repair.