I've got three external HDDs, all My Books, 500 GB, 750 GB, and 1 TB. If I hook one or two of them up to a computer, everything seems fine. If I hook all three of them to a computer (I've tried this on two different computers, one running Windows XP, one running Windows Vista), the bootup time goes up by more than an order of magnitude. It can suddenly take about 10-20 minutes to boot the machine, whereas before it might take a minute.

All three drives work fine on their own. I'm not using any kind of hub; all three are plugged directly into the machine.

Is this associated with some kind of inherent limit in USB? Is this bad hardware design in the CPU box? Is this a My Book problem?

  • Are the external drives active during this time ? I remember my system had the tendency to scan the external drives on boot. When does it take time, exactly ? Before, or during loading Windows ?
    – Gnoupi
    Aug 31, 2009 at 13:46
  • 2
    Not a solution, but there is a limit of USB devices. You can have 127 devices on a single bus. And you probably have more than one bus on your computer, so you aren't really hitting this limit. Aug 31, 2009 at 15:47
  • I've got more than 3, no problems. Aug 31, 2009 at 16:36
  • Typically, a motherboard will assign 2 USB ports per bus. So if you've got 6 ports, then you probably have 3 buses.
    – David C.
    Jul 22, 2013 at 21:12
  • So far as I am aware there is a bug in either windows 8 or the intel driver. Microsoft say it's the driver, intel say it's Windows. Regardless of how many hubs and how much power you have available once you reach about 16 connected devices problems will start and once you exceed it you will get errors and parts of your USB systems will shut down. Unplugging something will normally bring the remaining devices back up but they may need to be unplugged and plugged back in. This is a known issue and no-one wants to take ownership. For some you may find a driver for your chipset direct from Intel m
    – user459530
    Jun 16, 2015 at 18:31

5 Answers 5


If your BIOS is set to scan for USB boot devices on startup, more USB hard drives become more USB boot devices to scan before booting from the hard drive.

Try setting your BIOS to boot directly from the Hard Drive without scanning other resources first.

  • Hmm. Interesting...but still, shouldn't that increase things only by some linear factor? I'm seeing a tipping point where the boot time increases by a significantly larger percentage adding drive #n than it did by adding drive #(n-1).
    – Beska
    Aug 31, 2009 at 14:13
  • Since BIOS resources are limited, it's possible that once over a certain limit, the BIOS will have to allocate more resources than anticipated in order to scan the inordinate number of drives.
    – jweede
    Aug 31, 2009 at 14:28
  • Good theory, so +1, but in my case it didn't pan out. BIOS is apparently not scanning USB for bootable devices.
    – Beska
    Sep 1, 2009 at 12:55
  • no good. Guess you might want to see what software both computers have in common, to see if there's a program responsible.
    – jweede
    Sep 1, 2009 at 13:50

The only problem I've seen with multiple USB devices has been a power limit on the hub. Typically the USB ports on the computer will be linked to an internal hub. If you have ports on the front and back, have you tried spacing them out more? Say, 2 in back, 1 in front?

  • Power to the usb devices is something to consider and be aware of. Aug 31, 2009 at 13:01
  • I have a WD external drive connected to my IBM thinkpad. The computer will not power on with the drive connected because it sucks too much power out of the USB. If I connect after boot, all is well.
    – Michaelkay
    Aug 31, 2009 at 13:15
  • Would it then be a reasonable solution to try to have a single powered hub that I plugged all of these devices into?
    – Beska
    Aug 31, 2009 at 13:28
  • (And yes, I've tried some different connection schemes...though I'm inherently limited in what I can try...a limited number of ports, of course...some of which are already filled by keyboard, mouse, printer.)
    – Beska
    Aug 31, 2009 at 13:29
  • You might want to try spreading the heavy use devices out more and moving some stuff around so you don't have all 3 drives on the same hub. You could also try waiting until the computer has finished booting before hooking up the drives.
    – Dustin
    Aug 31, 2009 at 15:28

The official limit for USB devices is 127 per controller... In theory, you should be able to plug hubs into hubs into hubs (they will each need their own power source) until you have 127 devices total.

This is the printed technical limitation from back when I was in college, but I've never seen anyone actually using more than 12 ports simultaneously.

As for startup times, yes... Having too many devices plugged in can cause a slow start-up. The PC will need to run its usual startup process, then mount each drive, etc...

If you aren't using all the drives constantly, I would leave them all unplugged and connect them as you need them. USB is hot-pluggable, so make sure the power switch is in the off position, plug it in to the wall and USB port, then turn the power on. This will keep your computer fast, save electricity, and extend the life of your drives.

For more info on the USB tech standard, you can check Wikipedia

  • 256? I thought it was 127 and the hubs use up a slot. Aug 31, 2009 at 16:35
  • You are correct... Edited to reflect per en.wikipedia.org/wiki/Usb Aug 31, 2009 at 17:04
  • There is also a USB limit of 5 hubs. Meaning, if you connect 6 hubs to each other and then to the computer, the farthest hub will not be functional.
    – Rucent88
    Jul 22, 2014 at 4:34

While it is quite normal for the boot time to increase with a growing number of attched USB hard drives (by a few seconds), 10-20 minutes is WAY out of character.

And no, you can slap on as many drives you like until you run out of drive letters.

PS: I'm using four external disks permanently connected. They add about 15 seconds to the system's regular boot time (Windows XP, that is).

  • You don't even need driveletters; you can also mount drives inside directories. E.g. I've got a card reader that's C:\SD, C:\MMC and C:\CF.
    – MSalters
    Aug 16, 2011 at 7:58

Are your drivers up-to-date?

Perhaps the system is running a file system check on the drives? Are they being unmounted properly at shutdown?

Anything in your system log?

  • Drivers should be up to date, but I'll double check. Don't think that the system is running a file system check. Powerdowns are always clean. Will check the system log.
    – Beska
    Aug 31, 2009 at 13:00
  • 1
    Drivers should not be an issue as this is pre boot. Aug 31, 2009 at 14:43

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