If you’ve got a misbehaving Mac, an incredibly awesome tool to have in your arsenal is a program called EtreCheck. It’s free, though Etresoft is currently asking for donations to help improve El Capitan compatibility.
What this app does is a pretty thorough system scan, and it’ll report any problems it finds. To check it out, first download the program and launch it. Then it’ll ask you to choose which problem you’re having, if any.
Pick from that menu, choose “Start EtreCheck,” and it’ll run.
When the process is complete, your report will appear, and there’ll be just a ton of info, including hardware details and battery health:
…the status of your Time Machine backups, plus a lot of details about their configuration:
…and what’s taking up your memory.
It’ll even assist you in identifying certain types of system files that are in use if you’re doing more advanced troubleshooting.
Plus, there’s a great big ol’ “Share Report” button in the toolbar, so if you’re trying to help less tech-savvy folks with their computers, it’s just plain handy that they can send you a file where you can see all of that data at a glance.
As the developer notes on the Etresoft website, the info you get from EtreCheck can be used to post on the Apple Support Communities forums if you don’t feel comfortable deciphering its output yourself. But in any case, I think EtreCheck is well worth trying out, especially if your Mac isn’t doing what it’s supposed to!