For what it's worth it sounds like the problem is on Tascam's end and not SONAR.
I think I would try this process one more time and cross my fingers:
1. Unplug your tascam unit from your computer
2. Uninstall the tascam software
3. Reboot
4. Reinstall tascam software. Plug back in USB.
5. Reboot
Good luck to you. Hang tough with this forum and these guys will work hard to get you sorted out.
Also, you might post this same problem over in the hardware sections. There are guys that hang out over there that may be able to offer you help too.
All the best.