I sometimes get this , do not know why. I found if I range a small bit of the timeline (bits with no rendering req) and then do "Print To Tape" wait for the colour bars and then press cancel and all works well.
I have found on the laptop if yuou have an XMD & "AMD PowerNow Technology" if this is set to Automatic the output on firewire is erratic and not usable, you need to set to "High Performance Mode" and then all is well