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The reason you are seeing this message when you try to scan from your computer is that it is expecting to see a USB connected scanner. With most devices including this one, you cannot initiate a scan from a computer on the network to a networked printer. Make certain that you have the latest version of software for the printer under Yosemite. There are some limitations with scanning under the Mac operating systems that have come about because of the lack of scanning support in some of the drivers.there is some third-party software that is available to bridge this gap – it is called twain sane. You can download the software at the following page