How exactly is the Tascam connected to the Mac? Are you using a USB hub?
The Tascam requires sufficient power from the USB connection to allow the Mac to see it, so if you are using a USB hub then remove it and connect the Tascam directly to the Mac. Also remove any other unneccessary USB devices from the Mac.
Let me know if that helps?
Hi, my name is Rusty. I'm just now seeing this question. I'm a sound engineer, do you still need help with this?