iMac/iTunes no longer recognizes 1st gen iPad

My iMac running OS X Yosemite v 10.10.3, iTunes v 12.1.2.27 will no longer recognize my 1st gen iPad (64GB w/ 3G) running v 5.1.1 (9B206). The iPad goes nuts, and the iMac/iTunes repeatedly displays a message saying it cannot recognize the iPad. Plugging in to USB port, tried 2 different cables with same result. Plugged it into my work Dell laptop running Windows 7, and iTunes recognized it immediately. What can I do to start getting iTunes to sync my iPad again?