First Aid Fails on External Disk, what can I do from here? History:   Running an iMac late 2011 and updated the OS to Yosemite when the computer stopped booting. I have a history of killing HDD so I figured it was a bad HDD and I went to change the drive. Before I changed the drive I wanted to try and recover some of the files and using my Macbook I set the iMac in Targeted disk mode and I was able to access the drive and copy some of the files. Due to the length go time it took I shut down the computer and tried to pick up where I left off on the next day. This time target disk mode did not work. Since I had copied most of the important stuff I decided to change the drive and try and clone the the drive if it would spin up. 

I extracted the 1TB drive and cloned it using a disk duplicator. I plugged the Dock into the Macbook and Both HDD would spin and Disk Utility acknowledged their presence but would not mount either drive. 
    I figured it may be a conflict because they were clones and I shut down and removed one of the drives Drive 1 (Original iMac drive) Using a program called DriveDX I can monitor the overall health of the drive. Drive 2 is Status OK and GOD overall Health and GOOD performance rating. I tested Drive 1 and got similar results. I also ran self test with good results.
    I ran the First Aid on both Drive 1 and 2(Disk Utility) Both drives resulted in the same message:

"First Aid process has failed. If possible back up the data on this volume. Click done to continue."

repairing file system.
Checking journaled HFS plus Volume.
Checking Extents overflow file.
checking catalog file.
Missing thread record (id=18438430)
Missing thread record (id=18441061)
Missing thread record (id=18441109)
Missing thread record (id=18444964)
Missing thread record (id=18543123)
Missing thread record (id=18549821)
Incorrect number of thread records
Incorrect number of thread records
Checking Multi-linked files.
Checking catalog hierarchy.
Invalid directory item count
(it should be 59 instead of 77)
Missing thread record (id=18438409)


I will spare you the list but it goes on for several pages. Skip to the bottom and it says:

Checking extended attributes file.
Invalid node structure
The volume Macintosh HD could not be verified completely.
File system check exit code 8.
Updating boot support partitions for the volume as required.
File system verify or repair failed.
Operation failed...


From what I can tell the overall health of Drive 1 and 2 are good they both spin freely but neither will mount to the computer which appears to be a file structure issue??  How can I repair the drive or can I recover the drive all? I am not willing to spend $$$ on Data recovery because I was able to get most of the important stuff off the drive when I had it in Target disk mode. At this point I am just puzzled by the failure in the first place and would like to try and understand what happened and how to fix it.
Invalid node structure is bad news. You should backup as much data as possible then try repartitioning the drive using Disk Utility. 

If the repartition fails or the same issues crop up again it'll be time to replace that hard drive.