Problem booting into Windows 7 when external hard drive is plugged in

[Rustys]

You could wipe the drive and put it to NFTS.

Would use Diskpart to do that (or you can use Drive Manager).

Once the drive is done see how it runs DO NOT make any Chages to the properties otherwise you may end up right back here.

The problem must therefore be in the bios?

Other than the Seagate being the only one affected doubt it.

Could try clearing the CMOS
and or
Reset the BIOS to factory.

After you delete the partitions and before you wipe the drive would suggest that you delete all references to the drive in Device Manager.
(prior to reconnecting the drive)

[Robo7]

You could wipe the drive and put it to NFTS.

Would use Diskpart to do that (or you can use Drive Manager).

Once the drive is done see how it runs DO NOT make any Chages to the properties otherwise you may end up right back here.

Other than the Seagate being the only one affected doubt it.

Could try clearing the CMOS
and or
Reset the BIOS to factory.

After you delete the partitions and before you wipe the drive would suggest that you delete all references to the drive in Device Manager.
(prior to reconnecting the drive)

Thanks for the recommending wiping the drive with Diskpart and formatting it to NFTS. I did that and I can now reboot keeping the USB cable in, so all sorted, thank you very much!

Problem resolved.