tascam is extremely picky about what firewire its connected to.
it must be firewire 400 with TI chipset and the card can not be a combo card (has 400 and 800)
many times even onboard Ti firewire can be an issue
It doesn’t matter if it worked together with SX3 and XP. That doesn’t prove anything about C6/Win7 combination. It still MAY (while I doubt it, it MAY) be an incompatibility with your DM’s FW-card and your DAW’s FW interface. Please follow JCschild’s advice and check out your DAW’s FW chipset.