Mac Pro (Late 2013): Issues connecting a Thunderbolt device to another Thunderbolt device
Plugging in a Thunderbolt device to a Thunderbolt peripheral, such as a Thunderbolt Display, can cause a pause in connected devices. This can interrupt printing, scanning, or other operations on Thunderbolt and USB devices which are not designed handle a pause and are connected to that series of devices.
Avoid adding Thunderbolt devices to a Thunderbolt series while printing, scanning, or when other processes are taking place related to that series of devices.
Sometimes restarting the affected application will recover the devices, but you might need to disconnect the devices and plug them back in. Contact the device manufacturer to see if a software update is available that addresses its ability to handle a pause.