How do you verify a start up disk is not full?

There are many ways to get this information.

1. Right click on your desktop.

2. Click SHOW VIEW OPTIONS.

3. Check on SHOW VIEW ITEM INFO.

Now you can see the size of your HD and the remaining free space at all times. I keep my settings this way, easier to glance at.

In the Finder use 'Go Menu > Computer'

In the window that opens select the boot disk (it may be called 'Macintosh HD').

'File menu > Get info'

Capacity, Used and Available are listed in the 'General' section.

You want at least 20GB free for a stable OS. More free space can help older disks perform better. You should never run a Mac until the startup disk is full.