Q: Start-up disk full????

The startup disk is the hard drive your computer boots to. Can you see an icon for it on the desktop? If not, click on the desktop so the title next to the Apple icon at the upper left says Finder. Then choose Finder > Preferences. You should see this:

If it's not checked, but a check next to Hard disks. All mounted drives will appear on the left side of the screen, assuming you don't already have your desktop cluttered with files and folders. If more than one drive icon appears, and you're not sure which is your boot drive, go back to the Apple at the top left and choose About This Mac.

You'll see a heading for Startup Disk, followed by the name of the drive which is currently the boot drive (startup disk). Close the "About This Mac" window. Now locate that same named drive icon and right click on it, then choose Get Info from the menu that appears. What does it say next to Available?

Move items you don't need on it, such as the iTunes and iPhoto libraries, to another drive, verify they work in the new location, and then delete them from the internal drive. If you don't need something at all, drag it to the Trash and empty it.

(87632)

As Niel noted. You need to make room - lots of it. You only have a half GB of space left on the hard drive, and OS X should have at minimum a breathing room area of 2 GB.

Move any personal data off the drive onto an external hard drive that does not need to be on your main, internal drive. Videos take up huge amounts of space. Especially full length movies.

There is a real danger of the drive getting so full that the OS cannot operate properly, which can lead to data loss. So before you add any new files to the drive, move as much off as you can.

1. Select the folders containing them in the Finder and choose Get Info from the File menu.

2. Not if the external drive is plugged into the computer.

(87636)

Also as Niel said, they're just as accessible on an external. It's just a matter of turning the drive on.

You can't put immediate convenience ahead of stability, or possible file loss. Making room on your drive is imperative in its current state of capacity.

No; you shouldn't have both your primary and backup copies of anything on a single drive, as it can fail and cause the data to be lost.

(87638)

If you mean you're using it as a Time Machine backup, that's not the same thing.

1) A Time Machine backup leaves copies of everything on the main drive, so the external drive files are duplicates, which is what's supposed to happen. But...

2) If you remove the originals from the main drive, Time Machine will eventually delete the backup copies when it needs to make room for new data. Since the originals are no longer on the main drive, the assumption is you no longer need the copies, either.

For long term backups that will not automatically disappear, you need to put them on a drive that is not controlled by Time Machine.

Any external drive you want purchase. When you first plug it in and turn the drive on, and the system asks if you want to use it with Time Machine, just answer no. Then format it as Mac OS Extended and use it as a plain ol' hard drive. What you put on it stays there until you delete it. Once you copy your large data files to the external, they can then be deleted from the main drive.

Maybe Niel knows, but I don't use iPhoto or iMovie, so I'm not sure what happens when you manually move their databases of file and folders to another drive. They are there on the external at that point, but the apps will likely wonder where they went since it will looking for them in their original location.

The free app OmniDiskSweeper will locate and list files on your drive from largest to smallest. Do not delete items without knowing what they are!! After running the app, please list what it found here.