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Level 3

660 points

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

I'm confused about data transfer speeds. I've got a USB 2.0 hard drive that is supposed to have a data transfer rate of "up to" 480 MBps, and a FW800 drive that is supposed to have a transfer rate of "up to" 800 MBps. However, I tested reading and writing an 8.5 GB folder with 5,700 files in it. The ACTUAL transfer time was 8:52 for the USB (works out to 16 MBps) and 4:02 for the FW800 drive (works out to 35 MBps) - both substantially below the 'advertised' rate.

So I looked at some Read tests for hard drives, irrespective of interface. [ http://www.barefeats.com/hard56.html ] Apparently a 5600 rpm drive from Seagate or Hitachi only has a read speed of 36-37 MBps. (And the 'maxed out' 7200 rpm drive is not that much faster, at 46MBps). That tells me that my FW800 drive is achieving nearly maximum transfer speed. But now I don't understand why the USB 2.0 drive would be half as fast, if its 'claimed' speed of 480 MBps is still 10x faster than the maximum read or write speed of a 5600 rpm hard drive?

Is something wrong with my math?

MacBook Pro with lots of extra stuff, Mac OS X (10.5.1), iPhone in pocket, iPod in car, Nikon and Adobe everything

Posted on Feb 8, 2008 5:17 PM

Best reply

Peter Arnold1

Level 5

7,363 points

Posted on Feb 8, 2008 5:52 PM

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.

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22 replies

Kappy

Level 10

361,741 points

Feb 12, 2008 7:10 AM in response to J Michael

Bridge cards are not the same. FW400 and FW800 require different bridge cards. The speed of the hard drive is not the LCD (although it could be depending upon the drive.) I have hard drives with average throughputs approaching 80 MBps. I also have notebook drives that are half that and even less, although one of them is nearly as fast as the full-sized drives. Here's some quick results for large block sequential read/write of a 2.5" Hitachi notebook drive (7200 RPM - 200 GB):

FW400 R/W - 35 and 32 MBps
FW800 R/W - 58 and 53 MBps

That's for the same drive in the same enclosure (dual interface.)

I then tested a 500 GB -7200 RPM Maxtor full-size drive that's in a FW800 enclosure:

FW800 R/W - 66 and 54 MBps

Lastly here are the results for a Maxtor 300 GB - 7200 RPM drive installed in my Mac Pro - direct SATA 1 interface: R/W - 64 and 70 MBps.

All results using XBench which is not the best program for such tests, but the results are useful for comparisons.

Both drives are SATA 1 with interface maximum transfer rates of 150 MBps. Real world throughput rates are much slower, but the difference between the two large drives isn't much on the read side for the SATA and FW800 interfaces. For the small drive the difference between FW400 and FW800 is about 70%. The enclosure uses the Oxford 922 chipset.

You can draw what conclusions you may from this. I would say that FW800 is much faster than FW400. It also appears that real world performance for a SATA 1 drive isn't much different whether it's in a FW800 case or connected directly to the computer's controller. In either case performance isn't near theoretical maximums. I suppose we learn not to pay much attention to interface transfer rates as a guide to hard drive performance. And, FW800 can provide pretty decent performance.

Will-Hi

Level 5

4,307 points

Feb 12, 2008 7:40 AM in response to Kappy

Kappy wrote:
I would say that FW800 is much faster than FW400.

Which can be verified by doing an initial Time Machine backup with FW400, then doing the same backup with the same drive connected via FW800 😉

Kappy

Level 10

361,741 points

Feb 12, 2008 7:53 AM in response to Rod Hagen

I did a test quite some time ago of a SATA notebook drive connected to a Sonnet Express Card in a MacBook Pro and compared it to the same drive connected via FW800. Found no significant difference in performance. Furthermore, the Express Card required a driver (which can become incompatible with OS upgrades) and it was not bootable. I don't use it.

J Michael Author

Level 3

660 points

Feb 12, 2008 8:58 AM in response to Kappy

Thanks for the analysis. What this all boils down to is that I'm looking for two 2.5" external drives - one that I can use as a bootable clone, and one for my digital media. So far, the only makers that provide FW800 are LaCie and OWC, and both at a considerable price premium. So I'm wondering if a FW400 drive would be a good compromise. I currently have USB2.0 2.5" drives for both, and although I can boot off the drive in the case of an emergency, the boot time is excruciatingly slow.

I'm leaning towards the LaCie FW800 but am open to suggestions.

Kappy

Level 10

361,741 points

Feb 12, 2008 9:14 AM in response to J Michael

I never buy branded systems. I buy a bare drive and an enclosure. My Hitachi 7K200 is mounted in a MiniXpress TBS120 enclosure that provides FW400/800 and USB interfaces using the Oxford chipset. The enclosure comes from TransInternational for $99.00. The model number is now miniXpress825.

OWC has a nice enclosure for around $83.00. You can also get the drive from them at fairly good prices. The 200 GB Hitachi is $190.00.

If you can use a screwdriver you can put the drive in the case. Takes 5 minutes and is probably cheaper than buying a branded system. Plus you get to choose what drive you will have. Most branded systems give you the cheap drive.

J Michael Author

Level 3

660 points

Feb 12, 2008 9:41 AM in response to Kappy

I bought at OWC enclosure but didn't like the 'feel' of it. I know that is entirely subjective, but these things live in my briefcase. I have to like them. Also, I'm looking for a minimum 3 year Warranty. I'll look at the enclosure you are recommending.

I'm quite familiar with the 'screwdriver,' having installed a Hitachi 250GB drive into my MacBook Pro. That was fun!

Kappy

Level 10

361,741 points

Feb 12, 2008 11:35 AM in response to J Michael

The particular one I linked is similar to an enclosure I have that I purchased a long time ago from California Drives. Very well-made clear plastic and sturdy. Of course mine is only FW400 because when I bought it that's all there was! It's an excellent enclosure and I'd buy another if I needed one. However, I also have two of the miniXpress enclosures and they, too, are very well made. They are all metal.

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