I'm a Windows user and am not familiar with Mac.
Standard troubleshooting procedure is:
Roll back software updates
Try a different input cable, USB, firewire or similar
Update the driver
Uninstall the device software, wipe the drive free space, reinstall the software
Take the device to a repair shop if the previous steps do not correct the problem.
I assume you're using Epson scan or Silverfast in transparency mode with the document blocker in the lid/transparency unit removed.
Has the transport lock been set by accident? There is one for the bed and one for the transparency adapter.