My Internal HD won't boot 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.
I'm assuming that the partition Backup OS X on your external drive is a bootable partition.

Turn on the Mac and hold down the option key. This will show you all the bootable partitions that the Mac can find. If the partition Internal HD in your internal drive does not show up then EFI has not discovered the required files for a boot partition and the OS on it can be considered damaged and should be replaced.

From what you describe I suspect some sort of damage to the low level formatting of the partition.

At this point I would boot from another partition and use Carbon Copy Cloner or a similar tool to backup the damaged partition and then erase and reformat the entire drive before installing a new System or a known good backup.