Friday, 11 January 2008

External Hard Drive Data Rates 

Just a quick post on comparative speeds of various ports. The received (incorrect) wisdom in much of the PC world is that USB2 is faster than FireWire, because 480 Mbps is faster than 400 Mbps. However, potential maximum speed isn't the whole story. Because USB is a host-controlled bus, the CPU has to manage the data transfer while FireWire manages itself. Some good transfer rate/CPU requirements are found on this page.

Data for a Western Digital My Book Studio 500GB external drive with FW800, eSATA and USB connections; FW400 speeds are taken using a 9-pin (FW800) to 6-pin (FW400) cable:

USB2 averaged 29.0 MB/sec (15.0% CPU)
FW400 averaged 37.9 MB/sec (2.1% CPU)
FW800 averaged 61.5 MB/sec (3.7% CPU)

These numbers were (apparently) taken on a PC and historically USB2 has not been great on the Mac. Not sure if it has improved recently, but USB is always the poorest performer on a Mac. Probably more important is that you can daisy-chain one FireWire drive from another, so a single FireWire port can take as many drives as you care to throw at it. USB drives can't do this, so you'll need some powered hubs to connect many drives.

DV/HDV has lower data rate requirements than any of these ports should provide (~4MB/sec), but your minimum speed can never drop below the rate the video is arriving or you'll get dropped frames and likely an aborted capture. On the flipside is that some people report dropped frame problems with some Canon cameras and FW800 drives. I've had no issues capturing from my HV20 to my My Book Studio 1GB through FW800, but your mileage could vary.

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