What does a big question mark mean in the upper left hand corner of finder window? Appears on all my finder windows. I click on it and it says "The Item Can't Be Found." Very odd. I do use a lot of different external drives for backups, etc etc. Not sure it if is trying to reference one of those or what........??
It means you have an orphaned alias, one that has lost the path to the original file either because the original was deleted or moved. Drag the "?" off the toolbar and "poof" it.
I think MacJack is thinking it is in the Finder window. However, do you mean it is on the toolbar of a Finder window? If so, haku64 has the right method of removing it. But, if it is a tool you want to repair, right-click on the Toolbar and select Customize Toolbar… Either drag back the standard toolbar, or replace the malfunctioning item with a new one. You can then just drag the ? one out of the toolbar.