Are USB2.0 drives really 480Mbs?

By terabyte
Jan 26, 2005
  1. Hello people, this has been bothering me.
    I want to buy an external hard drive and found some external USB2.0 drives claiming speeds of upto 480Mbsp. However, the drive in the external casing is only 133ATA or 133Mbps. How does a 133 drive let data get out at 480?

    Also my other question is this: If 133 is Mega bytes per second, that should mean I can copy a 600Mb cd in about 6 seconds? I am not able to do that yet. Any explanation would be greatly appreciated. Thanks!

  2. SNGX1275

    SNGX1275 TS Forces Special Posts: 10,689   +395

    In theory yes, in reality no. I forget exactly what the explanation is but firewire for drives is better than usb2.0, something to do with information going both directions. Anyway, in the scope of your question, no its not going to speed up your drive any.

    A number in Mbps is Mega Bits Per Second. Not Mega Bytes Per Second, right there is the explanation. 8bits=1byte
  3. Nodsu

    Nodsu TS Rookie Posts: 5,837   +6

    The interface speed is just marketing. And yes, there is a difference between B and b :p

    Modern hard drives almost never reach ATA100 speeds, the effective transfer rates being some 50-80 megabytes per second at best.
  4. kritonas

    kritonas TS Rookie Posts: 49

    USB 2.0 is 480 megabits per second, and 8 bits make one byte.Therefore 480 mbits are equal to a speed of 60 mbytes per second. But that is only theoretical, you will never transfer at 60mb/s through a usb device.
    Hard disks never fill their link, for instance an ATA133 hard disk, will never transfer with 133 mbytes/sec simply because it can't read/write with that speed, therefore it just uses part of that bandwidth.
    Now, a usb2.0 hard disk limits an ATA 133 hard disk to 60mb/s, so that will have an impact on the disks performance. plus, 60mb/s cannot be achieved practically from a usb device, because there are other things that limit the speed(for example the usb interface relies a lot on software, thus it needs processing power.)
    oh well.
    correct me if i'm wrong.
  5. terabyte

    terabyte TS Rookie Topic Starter

    I must have been mentally drunk yesterday night when I was writing about MegaBytes. Of course 480 is mega bits and not bytes. Well now, thanks everyone for explaining it to me. Now I have another question along the same lines: I have a network device (a Snap Server with Raid-0 on two ATA133 Maxtor 250GB drives with 8MB cache each) that has a 100mbps Ethernet interface. When I save 5 Gigabytes to it (via a Ethernet 100mbps Hub) , it took me almost 120 minutes. However, when I saved the same 5 Gigabytes to my external USB2.0 ATA133 drive, it took me just about 25 minutes (A fourth of the time approximately). I really would like to know why my USB2.0 external drive can save data 4 times faster than my 100mbps ethernet device(the Snap Server). That is what triggered me into thinking.
    Since 4 times 100(as in ethernet) = 400 (as in usb2), I was wondering if the USB2 drive was really transferring at about 400mbps, but then the 133 ata is getting me lost.

    Thanks again for ideas.

    Always ready to learn something new.

  6. kritonas

    kritonas TS Rookie Posts: 49

    You are confused because ATA is measured in megabytes:)
    Therefore the actual bandwidth of your hdd is 133mbytes/sec (that is 1064mbits/sec).
    that is limited by the usb 2.0 to less than 480mbits/sec. while your lan has a bandwidth of less than a fourth(100mbits/sec).
    Dont forget that the lan has other factors you must consider, such as latency, and collisions.
    I hope what i said is understandable:p
  7. terabyte

    terabyte TS Rookie Topic Starter

    Aha. Now I am in business. The ATA really threw me off. Thanks for clarifying or I would be in the dark until light fell on me. I was considering all labels as mega bits rather than mega bytes. Now that I read your post and understand that ATA is measured in megabytes, everything is falling into place and I am not lost anymore.
  8. Rick

    Rick TechSpot Staff Posts: 4,573   +65

    MB and Mb make a big difference.

    While the USB 2.0 interface provides 480Mbps, the actual media itself for those fancy flash pens is lucky to get past 20Mbps. :zzz:

    I think that's a good example of how gimmicky marketing is anymore. Everyone's busy throwing out big numbers and it's your job as the consumer to crunch those numbers... Didn't use to be that way... As much. :)
  9. MrDJ009

    MrDJ009 TS Rookie

    Step up to Gigabit.

    Now that we're clear about why Fast Ethernet, at 100 Mbps, can sometimes be slow, it might be time to consider stepping up to Gigabit Ethernet at 1000 Mbps. Gigabit switches and network cards can be had for very reasonable prices. Upgrading the network would improve PC <--> server backup times, since the network is a bottleneck.
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