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Laptop is Acer TravelMate 331T.
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It is the "Easy Link" Combo Drive port.
From the service manual "The EasyLink™ Combo Drive is a two-in-one floppy/CD-ROM or floppy /DVD-ROM “combo” drive that connects externally to the computer" Picture of drive is here: Acer EasyLink Drive
Good manual here Acer Service Manual
Required, but never shown