Officially, it’s the “spinning wait cursor” or the “spinning disc pointer”. Colloquially, it goes by many names, including the Spinning Beach Ball. Whatever you call it, the colourful pinwheel is not a welcome sight.

According to Apple’s Human Interface Guidelines, “the spinning wait cursor is displayed automatically… when an application cannot handle all of the events it receives. If an application does not respond for about two to four seconds, the spinning wait cursor appears.” Which is to say, the beach ball is there to tell you that your Mac is too busy with a task to respond normally.

Usually, the pinwheel quickly reverts to the regular mouse pointer. When it doesn’t go away, it becomes what some call the Spinning Beach Ball of Death (also known as the Marble of Doom). At those times, it helps to know why the thing appears and what you can do to make it go away.

If an application keeps crashing, your best tool for finding out why is Mac OS X’s own Activity Monitor

Hardware causes
It’s not unusual to see the beach ball when your Mac is performing complex computing tasks. Even everyday tasks – like syncing large files with iTunes – can overtax the central processing unit (CPU).

To find out if the CPU is the bottleneck, open Activity Monitor (/Applications/Utilities) and, in the CPU pane, click on the % CPU column to sort applications by CPU usage. Keep an eye on that column. The processes consuming the most CPU cycles will be at the top.

The beach ball may also appear if you don’t have enough RAM or hard-drive space. Virtual- memory paging and swapping (making room in RAM by temporarily moving unneeded data from active memory to the hard drive) consumes CPU cycles. Insufficient RAM means more paging and swapping. Ditto if your startup disk is nearly full and has little space for swap files. In both cases, more swapping demands more CPU cycles, which means that fewer of those cycles are available to your applications.

Again, you can use Activity Monitor to diagnose these problems; click on either the System Memory tab or the Disk Usage tab. In the pie charts shown in these panes, more green is better. 

If you can isolate a hardware cause, the solution is obvious: upgrade. If it’s the RAM or the hard drive, you can upgrade those individually. In the case of the CPU, however, that means buying a new Mac. If those aren’t viable options for you, you’ll just have to run fewer applications concurrently; the more resource-intensive your applications are, the fewer you should run.

If your CPU is overwhelmed, Activity Monitor can tell you how much of its resources your programs are claiming.

The beach ball can also appear when you try to access your hard disk or optical drives (by opening or saving a file, for example) when they’re in standby mode. (That’s when they spin down after a period of inactivity to save energy.)

You can, if you wish, keep your startup disk from ever entering standby mode. To do so, open Energy Saver preferences (in System Preferences) and deselect Put the Hard Disk(s) To Sleep When Possible. Note that all your drives will still enter standby mode when your Mac enters its own sleep mode; you may see the beach ball if you wake your Mac and then immediately try to access a disk.

The Energy Saver option primarily affects your startup drive alone. Other drives (internal and external) may still spin down on their own schedules; that means you could see the beach ball if you tried to access them at the wrong time. Contact the manufacturer of the drive to see if a firmware update is available to improve the drive’s cooperation with Mac OS X’s power management.