I have attempted multiple times to update my iMac from the App Store updates section like I did when I updated to Mavericks, Yosemite, then to El Capitan. It would finish the download and then nothing. No attempt at installing the update.
Next I found the update in a dmg file at Apple's website. I would run the package contained inside but would get the error during the installation:
"OS X El Capitan Update can't be installed on this disk. This volume does not meet the requirements for this update."
What must I do to get my Early 2009 iMac updated?
(I am still learning about Macintosh computers.)