The pinwheel on a Mac, often referred to as a beach ball, is a rainbow-colored circular icon that appears in place of the standard cursor icon when the system hangs. More often than not, the pinwheel will appear for just a few seconds as the operating system catches up with the executed commands. Sometimes, the pinwheel means an application has completely stopped responding and must be manually exited.
Wait. The pinwheel typically appears as the computer is working something out, for example, opening an application or file, saving a file, switching applications. As the system catches up, the pinwheel should disappear after a few minutes.
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Quit the application causing the pinwheel. Right click the application's Dock icon and select "Quit."
Force quit the application. If the pinwheel has persisted for longer than 5 minutes and does not quit when you tell it to (as in Step 2), there's a good chance the offending application has stopped responding. In such cases, you must force quit the application. Open the Apple menu, select "Force Quit..." The Force Quit window will show a list of all running applications. The offending application will most likely be highlighted in red text accompanied by "not responding." Select the application in the list and click "Force Quit" to manually shut down the application. Once shut down, the pinwheel should disappear.
Add more RAM (random access memory). Increasing the memory on your Mac will allow the computer to handle more data simultaneously and thus reduce system hang ups.
Depending on the amount of memory your Mac has, it can only handle a certain number of applications being open at a time. Try quitting a few unused applications to prevent or minimize system hang ups in the future.