I have been using my US 144 for a couple of years with no problems until now. Everything is pretty straightforward using the USB connection to my computer, Midi, and analogue in and out. Searching the internet I have found this 'No Valid Signal' seems to be a common issue but all attempts at reinstalling software etc have failed. Some threads I have found seem to suggest there has been a Javascript update with Macintosh causing some problem with the Tascams. Some people have tried to get answers from Mac with no responses. I really do not have money to replace my unit with something else but for the moment I seem to be out of business with no fix in sight. Any help will be appreciated.

I have no fix, especially as you've done your homework and it's due to some changes in OS X. Maybe to write to Tascam and ask for an update would be of use.