Old IPAD syncing

have you read Use the Finder to sync your iPhone, iPad, or iPod with your computer - Apple Support

Suddenly I can't sync my 11-inch ipad pro (just updated to ios 14.7) with my MacBook (running Big Sur 11.4). When I hook up the ipad via USB cable, per usual, Finder shows it at first, lets me begin the backup... then the ipad disappears from Finder. Over and over again. It's not the USB port, because it worked fine syncing my older ipad just before I tried the newer one.