I just purchased a Seagate 750 gig external hard drive. My computer (running Windows XP home edition) will not boot past the screen where the devices are recognized as long as the Seagate is physically connected. I have to disconnect either the power cord or USB cord to the Seagate and reboot. Once the computer has booted, I can reconnect the Seagate cable and everything works great once the system recognizes the external drive. PLEASE...any suggestions?

Boot into your BIOS and change the boot Priority.

Make sure the USB boot Device is *after* your primary hard drive.

I tried entering SETUP with the external hard drive connected, but it still will not advance past recognizing the Pri Master, Sec Master & Sec Slave. The system just locks up at this screen.

Disconnect the external drive and reboot. Enter the BIOS setup and navigate to the setup priority (or setup order) screen.

While some BIOS's will tell the difference, some versions think a USB drive is a CD and will attempt to boot from it if CD is first in boot order (sometimes a manufacturer will leave it first after the machine setup). Make sure the hard drive is first in boot order, save, then try again.

I have the same problem mine is a lacie 750gb and I have I changed the boot order so that everything else would boot before the external would boot but still it won't go past the inital gateway screen( computer is a gateway)

Also at BIOS enable Usb Legacy Support.

I am all of a sudden having this problem on XP with my WD it has been working fine for months and now PC won't boot up with it connected. Any ideas plz?

This isn't an answer so much as an observation -- recently I've seen a number of XP systems with 5+ year old hardware have trouble booting with USB devices such as printers / scanners / external hard drives powered up. In the case of printers, we are suspecting issues with power negotiation on older USB ports with new devices. These all seemed to come about after Vista drivers were issued for these devices, which makes me wonder about the testing on all versions of XP (perhaps only one version rather than all). I would like to think an update to the driver for a WD device wouldn't have that issue, but it is possible. Might be worth checking to see if WD has a driver update in response to a MS patch, as well -- this can cause similar symptoms.

If this is a new HD purchased directly from retailer in the case already, i wouldn't recommend opening yourself (opening can void the warranty) unless you know what you are doing, but if the BIOS setting don't work, the HD itself has a jumper on it to set for master, slave drive, or cable select. You may be able to change this so it is set to slave and not being recognized as the primary drive and trying to boot windows from this drive, which it won't find not being installed on there, and will cause a blank screen, freeze or an indefinite hang in the system as the computer is looking for windows. This not being any help to those who have had theirs working before.

Had the same problem, searched the Internet and found a million people with the same problem and a million helpful people and their "solutions" to the problem, of which some did just not work, and some were downright dangerous to the health of my PC. But at last I found a solution that works, and it's simple (thank you to saykomatrixx):

First (of course) you check your BIOS to see that USB-devices are below (after) internal HD:s in the boot sequence.
My BIOS did not, however, have this option, only Disk drives, HD:s, Network, and Floppy(?!), but here's the beauty:
Find (in BIOS) the "USB Legacy Support", make sure it is set to "Enabled", and voilà, it should work like a charm to start-up with the external HD:s connected and on.

I did get a small surprise during next start-up when Windows started to install drivers for my sound card, and later I had to change some settings in my sound card to get my microphone to work again, as the driver install had changed some settings there, but otherwise everything is just hunky dory.

Good luck.

PS
I don't really know what the "USB Legacy Support" is/does (perhaps a more educated member could tell us?) but as my already was in "Auto" which is a sort of semi-on (software controlled is my guess), the difference can't be that huge from before I switched it fully on.
DS