is there a reason you are using DVCAm to playback and not playing back from a SSD or SCSI drive as a file?

did you upgrade the version/build of catalyst you were using on the Imac when you went over to the mac pro? if you were previously using a very old build of v4 (like beginning of last year) and you've gone over to the latest release then there are some differences in the video input set up.

otherwise it's possible that just a restart with the device plugged in from boot will sort it.

if you have any trouble please feel free to pm me

regards
Paul