Hello, I wish to know if it is possible to:
1-install openSUSE on external usb drive
2-boot from it using bios’ priority list of boot
3-don’t get involved with bootloader at any time.
Is it possible?
You can get your bios to select the external drive, but it’ll still need to find a bootloader.
The bootloader can be put on the external drive, and won’t affect the operating systems (or bootloaders) on the internal drive at all. It can be on the MBR of the external drive, or on the root or boot partition of an operating system on the external drive.
As far as I’m aware, it is impossible to boot an operating system without using a bootloader (someone may know different… but I’ve never heard of it done with Linux).
I hope it is possible to place the bootloader in the external hd, because i don’t want to corrupt my xp mbr at all…should i change external hd, should i not use it anymore, don’t want to keep a bootloader for no reason 
That isn’t a problem at all.
What you have to do is go through the installer, making sure at the partitioning stage to only place partitions on the external drive. You’ll probably want swap (1 or 2 times your RAM is probably OK), something like 15 - 25 gig for / (root), and the rest /home.
Eventually you’ll get to a screen summarising various sections. I think it’s just after you’ve selected which desktop environment and package groups you want on the DVD - not sure when it’ll be on the CD, but it should be pretty obvious - it’s sort of a big list of what it plans to do. Take your time, read carefully, and you can’t miss it.
One of these sections will be entitled ‘Bootloader’. Select to change that, and it will have an option asking you where you want to install it. Put it on SUSE’s root partition, and nowhere else - don’t select MBR, because I’m pretty sure it will put it on your main MBR, not the external drive’s one.
If it doesn’t boot, it’s probably because the drive order’s changed - that can be fixed. And if you mess up, and accidentally overwrite your XP bootloader, don’t panic, because it can be fixed.
Even safer: disconnect the internal drive. After finishing install and testing it, reconnect the internal drive. But, like Confuseling wrote, these days it can all be repaired, relatively easy.
thanks a lot all for your replies… i think i will try to install now