I have been issues with Time Machine off and on for a while now. I have tried using two HDs (one of which was brand new at the time) and have had issues with both of them. I was most recently using the older of the two HDs. I bought the new one when the old one started acting up. Then the new one started behaving oddly so I deleted everything off of the old one and was using that as my backup. I did not delete any files off of my new HD. Both HDs do not show up in Finder until they are plugged in for a long time (at least 30 minutes). When they do show up, it is with a warning about the HD (could not repair HD or could not backup to it). I would take this into my local Apple store, but the fact that I have to wait so long for the problem to appear makes this inconvenient. I ran First Aid on both HDs with Disk Utility and it failed for both HDs. See this album for error messages. The first three are the older HD that I was most recently using for backups. I am running 10.11.4 on a summer 2012 MBA. I also have some backups from my previous Mac (2007 white MacBook) on both HDs. What is going on here? The new HD started acting up a few months into its lifespan. I contacted Western Digital, the manufacturer, about their warranty and my drive is well within warranty. However, I want to make see if there might be something not related to the HDs that is causing my issues since it is happening with both HDs.