My Imac keeps saying my startup disc is full...

Check to see that you have those items elsewhere--meaning only the ones you want to keep--if you do you can delete the contents of Shared, but not the enclosing Folder itself. The only purpose of Shared is for other Users to have access to those items.

My Shared is empty, since my only other User account is an admin account I use only for authentication, i.e. giving an admin password when needed and for testing.

These are the exact files that I mentioned in my previous post that can be very large and take up all of your hard drive space.

You need to move these files to an external hard drive than delete them off of your internal drive.

There are several to do this

If you purchase a new external drive, you can use an app like Carbon Copy Cloner or Super Duper to copy/clone your entire internal HD to the new external hard drive.

This would create the potential for a bootable system and data on an external hard drive.

Then just delete the actual music, movies and photos folders off your internal HD leaving just the OS X system.

If you already use an external HD for Time Machine backups, you can't use that drive unless you can archive all of your data to optical DVD discs as you would need to completely erase and reformat that drive to create more than one partition on that drive. The first partition would be big enough to clone your entire internal HD and the second partition would be what is left of the remaining free space of the drive to store your Time Machine backups.

With another external drive, you would still need to do this, but you can still keep your Time Machine drive intact and still use for Time Machine backups.

Here's the proper way of transferring all of your iTunes and iPhoto files to an external HD.

Apple explains how to move those libraries in:

iMovie: How to move the Library to an EHD

iPhoto: How to move the Library to an EHD

iTunes: How to move the library to an EHD

If the HD you are using only has one huge partition, then using the drive for anything else will be a pain unless you can find a way to offload the data onto disc and then erase and reformat the drive to have two partitions.

That is why I recommended purchasing another external drive.

That way it will be much easier to erase and format the new drive with 2 partitions and then copy/clone your internal HD to the new drive and then delete the data off the internal drive.

One partition you can just install OS X and all of your applications. Make this partition approx. 2-3 times the size of your OS X system and apps. Keep the remaining partition and HD space for data storage such as for your iTunes, iPhoto, and any other project libraries.

Consider purchasing, at least a 1 TB drive. 2 TB drive would be ideal as you'll know you won't run out of storage for quite awhile.

This is not a pain once it is setup properly.

I have all of my iTunes, iPhoto, iMovie and Final Cut and Logic projects all on external HDs.

For fast access to the ext. HD, stick with a FireWire 400/800 connection unless your iMac has Thunderbolt, then use that. Find ext. HD that uses FW or Thunderbolt connections.

Works great.

I understand.

Other places to delete or archive is old emails in Apple Mail.

Also, make sure the trash is emptied in the Apple Mail app.

Like iPhoto, there is a separate Trash in Mail that needs to be emptied.

Also, in your documents folder, delete or archive any old or no longer used docs.

Also, delete any type of "Read Me type files you do not use or read.

You may want to consider uninstalling any third party applications you no longer use or haven't used in a very long time.

Use a program called OnyX for your version of OS X.

Launch the program, let it do its initial thing, then find the maintenance and cleaning tabs.

Clear out all web browser and system caches. Delete all old system Log/error files. Clear out all web browser histories and, if you like, all web browser cookies.