I am new to linux and this is my first linux installation
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I have windows vista already installed on my internal hard drive and i installed mandriva on the external usb drive. Since I already had some data on the external drive, I created two partition and installed mandriva on the second partition. I also installed mandriva default(GRUB) bootloader to the first partition of my external hard drive. The OS ran perfectly for the first few times .ie. i was able to boot to mandriva form my usb drive by changing the boot device ,as well as to vista when I set internal drive as the boot device.

When I plugged in my external drive again and tried to boot form it, the boot timed out and the system booted from the internal drive (.ie. to vista).
The only difference which happened from the first few runs and now is the order of the usb drives. i have one more usb storage media and it was also plugged in during the initial installation and run.

Is there any way I can boot to mandriva from the externall usb hard drive the same way as it happened the first few times? I also want to be able to boot from the external HDD on any computer which supports usb booting to mandriva when I plug the drive in.

Thank You

I'm not sure but you can check me on this.
When you set the boot drive in BIOS, it becomes drive 0 to grub. There must be a bootloader in the MBR of your boot drive in order to boot an OS, so I think you must have installed grub to the MBR of the usb drive, no?
It might work to modify /boot/grub/menu.lst on your Mandriva installation to have the proper grub name for the root partition, be sure that grub is installed to the MBR of that drive, and set that drive as the boot drive in BIOS.

So, if the second partition, where you installed Mandriva, is the second primary partition, it would be (hd0,1) in grub speak, when that disk is the first disk. Usually, the installer will make an extended partition after there is already one primary partition, in order to preserve the limited number of primary partitions. So, the second partition is extended and the first logical partition is 5, (hd0,4) in grub speak.

If this analysis is correct, menu.lst can be edited from a linux livecd and grub run from that same session to install grub to the MBR of that disk. If you installed from the ONE livecd, you could use that. If you installed from the DVD, it has a rescue session where this can be done in console mode. If you want to try, I could advise, once knowing what you can boot with, linux-wise.

If you have more than one usb media device connected to your computer when starting, the BIOS will number the devices in the order it detects them. Sometimes, your external usb drive will be detected first, other times if you have a flash drive also connected, then it might be detected first. When booting from the external drive, make sure it is the only storage type usb device,and you won't run into that problem.

As far as connecting to other machines, you may need to reconfigure the xorg.conf file on some computers. And it also gets more complicated if you have enabled compiz during boot and configured it to run on an ATI graphics card, but the new computer has an nvidia card. If you do need those capabilities, it may be better to run OpenSUSE and have sax2 (OpenSUSE's video configuration application) to restart each time during boot.