I have several external hard drives. One of my drives contains an El Capitan beta, a QRecall back up, and a Windows backup as well. My Drive Genius Drive Pulse software gave me a warning that this hard drive was having integrity problems. I did some work on the drive and here is what I found.
I booted from my Tech HD and ran various utilities on the partitions which seemed to work okay. To finish off I ran the Physical Check program inside of Drive Genius. It found all kinds of hardware problems on the drive immediately.
When a hard drive has this many errors it is basically toast. Yes Drive Genius has a function that can repair hard drives but that experience has never worked out well for me. If you have a dying hard drive and are tempted to repair it, my advice is to forget it. Just buy a new hard drive. Hard drives are very inexpensive and it is not worth the bother to repair them.
You were probably wondering if the drive showed any external symptoms. After I thought about it I noticed that it had been slow to boot up and occasionally it would freeze up. The freezing part is a sure sign of a problem hard drive.
I happened to have a one terabyte drive on hand that was about one year old. I put this drive into a drive sled, partitioned it and used the SuperDuper! application to clone each partition over. I then put this hard drive into an enclosure and was able to see everything again. I was able to boot into El Capitan without any issues.
It is nice to have an extra hard drive around in case of failure. The Drive Genius application worked flawlessly, I highly recommended it to you.