Ok my last system was a XP 32 bit and I had a ext hardrive used to keep purchased addons,backups,sceneries,you name it. Then after a system lock up, I started getting the dreaded NTLDR missing.
Long story short, the computer was trying to "boot" off that ext drive. Going into bios showed the main HD missing and that ext hd was priority one. Its like the computer cannot find the actual main os hard drive so it instead uses the ext drive to try and boot off of.

Jump to new system I have. Its a I7 Vista 64.
Had a simular incident so I bought a new ext hardrive.
Well guess what, its happened a few times in the last month.Totally new machine and os. My system lockup wile online. Reboot and got the NTLDT missing. Unplugged the ext HD and all is well. Even if I edit bios and remove that ext drive from boot priorities, when this does happen, my main hard drive is missing and the ext hd is shown as boot priority drive 1.
My only solution I can find is to turn off the power to the ext drive when I am not using it.
Any help appreciated as I am afraid eventually I will not be able to boot up at all and do yet another os reinstall. Thanks, Jim

Note- This doesnt happen every boot. Only on occassion and never on first boot or after reawakening the system after being put on sleep. Seems like if I get a online page freeze,game lockup ect requiring a reboot or if I install a prog that reauires a reboot.
In bios the main os HD will be missing. If I unplug or turn the power off to the ext drive, the system starts normally and in bios, the os hd is there.