Their are many options today for you to attach your trusty old FireWire 400 goodies to your shiny new MacPro (that doesn't include FW400 anymore). 6 pin to 9 pin adapters, cables or just buy new gear...except you didn't plan that far ahead and now your in the middle of a critical project with one file stuck on a trusty FW400 drive and no way to plug it into your new Mac. Sure you could load it onto that old MacBook Pro you got, but the HD is too full to for the 65GB video file.
Simply connect your MBP (powered off) to your new Mac via a FW800 cable (9 pin to 9 pin). Then plug in your FW400 peripheral to the MBP and it will mount on your new Mac no questions asked! In a pinch the MBP can be used as a pass-through for firewire (converting from 6 pin to 9 pin) and it doesn't even need to be plugged in, or have the battery in.
I tested this with a self-powered HD enclosure, a Gen 4 iPod, and a JVC DVCam/Mini DV tape deck (4 pin to 6 pin) and all worked flawlessly.
[crarko adds: I have used techniques similar to this ever since Apple moved to FireWire 800 only machines. With both the 400-800 converters and also using those Macs that have both types of port as an intermediary.]
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