The math of data transfer: USB 2.0 and FW800 both faster than HDD speed?

One issue is that you are mixing up Megabits-per-second (Mbps) and MegaBytes-per-second (MBps). By convention, little "b" equals bits and capital "B" equals bytes. Firewire and USB are specified in Mbps while transfer rates of hard disk drives are typically measured in MBps. A byte is eight bits.

For example, FW800 has a maximum rate of 800 Mbps which is equivalent to 100 MBps. Similarly, USB has a maximum rate of 480 Mbps which is equivalent to 60 MBps.

On paper, both USB 2.0 and FW800 transfer rates should accommodate the maximum transfer rate of the hard disk drive. In practice, USB 2.0 is always slower because the USB protocol was never designed to accommodate long, sustained transfers. USB was designed for keyboards and mice. Firewire, on the other hand, was designed for hard drives from the get-go.

Correct. FW400 has a maximum line rate of 50 MBps which is greater than the 46 MBps rating of the 7200 RPM disk drive you described. However, if you were to use a disk drive whose transfer rate exceeded 50 MBps, then FW 800 would be beneficial.

Yes, so long as the transfer rate of the drive is less than 50 MBps. One advantage of buying a FW 800 enclosure is that, in the future, you could install a faster hard drive into the enclosure and realize improved performance. There are hard drives available today that have sustained transfer rates specified at 100 MBps.

Can I jump in?

Does number of devices in Firewire chain have an effect on speeds? If so then FW800, having more capacity, might support more devices before being limited.

One thing that I think should be emphasised here is Peter's comment concerning the less than optimal performance of USB 2.0 based drives.

The Mbps figures alone simply don't tell the story. A supposedly "480 Mbps" USB 2.0 drive connected to a Mac will a be much, much slower than a "400 Mbps" FW 400.

On older PPC based Macs the difference was quite staggering with USB 2 struggling to make even half the speed of FW 400, and operating at less than a third that of FW 800 in real world tests - see http://www.barefeats.com/usb2.html for example.

Things are a little better on the Intel based Macs, but, as the barefeats article at http://www.barefeats.com/hard69.html points out, USB 2 drives on Macs still struggle to reach 20 MBps, while even FW400 manages 36 MBps.

USB 2. performance is one area where, I hate to say it, many Windows machines beat Macs by a substantial margin, though even then it will usually be slower than FW400.

If you are looking for an external drive for your MBP then *look for one with at least FW 400, and grumble if your favourite supplier doesn't carry them.* Those cheap USB 2 drives might look like good value, and yes, you can even boot from them with current macs, but they will be as slow as the proverbial snail compared to a FW 800 , or even FW 400, drive.

The other option, these days, is an eSata drive connected through your Express Card port. With an appropriate fast drive fitted this will be the quickest option, though also the most expensive by a significant margin by the time you add in the necessary express card adaptor, and, of course, it takes up a port that you might wish to use for something else.

Cheers

Rod

PS , Camoracer, as I understand it FW is able to handle chained devices with less speed loss. With USB especially, slow downs are likely to be very dramatic (regardless of platform) if you combine "low speed" devices on the same USB bus as your HD and use them simultaneously.

Message was edited by: Rod Hagen

The Firewire protocol was designed to allow autonomous transfers between devices where each device gets its own slice of the bandwidth. USB protocol does not allow for daisy-chaining and works in master/slave mode where multiple devices must be hubbed with the CPU organizing the data flow.

So, in theory, Firewire is better at sharing bandwidth across multiple devices and will provide better throughput than USB devices sharing a datalink. I don't have any personal experience with multiple, daisy-chained Firewire drives. I've read that, in practice, the performance of daisy-chained FW drives is quite a bit less than the ideal. Regardless, I would expect it to be superior to hubbed USB drives at least to some extent. If nothing else, using Firewire doesn't put as big a load on the CPU.

You might want to Google on daisy-chained FW hard drive performance. I know some testing results have been published.

Thanks Peter and Rod. This has been really interesting. I have been using FW400 external drive for backup for many years on the belief they were faster. Never really understood why though.

Do bear in mind that the speed ratings for those ports are their interface transfer rates, not throughput rates. So although FW400 has an interface rate of 400 Mbps (50 MBps) that isn't the throughput rate which is usually much slower. Your hard drive has an interface rate of 150 or 300 MBps (SATA 1 or SATA 2) but throughput rates are usually closer to 50-80 MBps.

The maximum interface rate for FW400 is 50 MBps, and FW800 is 100 MBps. But throughput rates are a function of other things such as the bridge card used in the enclosure and the drivers as well as the size of the data packets. Throughput rates are therefore considerably slower. More in the order of 10-40 MBps for sequential read/writes.