Leopard not updating
On August 28, 2009, it was released worldwide, and was made available for purchase from Apple's website and its retail stores at the price of US for a single-user license.
As a result of the low price, initial sales of Snow Leopard were significantly higher than that of its predecessors.
Addition of new end-user features was not a primary consideration: its name signified its goal to be a refinement of the previous OS X version, Leopard.
Much of the software in Mac OS X was extensively rewritten for this release in order to take advantage fully of modern Macintosh hardware.
When you see a 'This version of Safari is no longer supported' message, it's time to upgrade your Mac.
Older versions of OS X don't get the newest fixes from Apple. If the old version of OS X you're running doesn't get important updates to Safari anymore, you're going to have to update to a newer version of OS X first.
With the release of Sierra, Apple decided to drop support for some Mac models that were supported by several previous releases of the Mac operating system.Manually downloading and installing an update such as 10.6.8 combo updater works as it should, and then check for updates no longer presents that particular update as an option.This seems to be the result of some setting or 3rd party application as I've got 3 out 7 computers experiencing this exact same problem. Try doing a Repair Disk and Repair Permissions in Disk Utility, just to be sure.After downloading and then clicking restart the computer sits at the purplish sky desktop with no progress bar, and then after about 3 minutes it goes back to the login window (without ever installing or restarting).If I then select check for updates the same updates will all be presented and I can repeat the process.