Unable to delete files from my startup disk; can move to trash and delete them there, but cannot delete from startup disk?

Question

I constantly get a warning message that my startup disk is full and I need to remove/back up files from it.  I have already backed up the files from my "All Documents" folder, which is on the left side of the menu in my startup disk.  The problem is, I cannot delete those same files from the "All Documents" folder on my startup disk.  It will let me move them to the trash, which I can empty, but they still do not delete from the "All Documents" folder.  Does anyone know why?

Answer 1

Drag the files you want to delete (this applies to applications too) from their current location to the Trashcan icon in the dock. The trash will appear full. Hold down the control key and click on the Trashcan and select "Empty Trash" from the pop-up menu.
Alternatives to the control-click method: double-click on the Trashcan and click the "Empty" button in the upper right corner of the window that comes up; right-click on the Trashcan (only works if you have right-click enabled); press shift-command-delete; select "Emtpy trash..." from the Finder menu.