I don't know why I can not either write the grub to my external usb drive's MBR (I tried many suggestions), but anyway, I tried to boot installed ubuntu 7.04 by using grub bootable Cd. (GRUB Manual)
I successfully made iso files and burned and boot from CD. I got grub menus (since made menu.lst file) and made it to boot ubuntu which is in USB drive, but it shows me error 25, Disk read error.
This is my hard disk
/dev/sda (XP, not usb drive)
/dev/sdb (ubuntu, usb external drive)
Before, I tried to install ubuntu with using installing GRUB to MBR of /dev/sdb, but after loading from that usb disk, it failed showing Operating System Not Found. Also, I tried to install grub to /dev/sda's MBR and install ubuntu and load from /dev/sda's MBR to /dev/sdb's /boot file but it shows me of error 25isk read error.
I don't know this is the reason, but in my one external usb drive, it has two SATA disks on it, but in operting system and GParted, it is showed as one usb disk (My disk is WD MY Book Pro Edition II 1TB edition).
Here is my grub.iso that includes menu.lst. Please notify me that if my menu.lst is not good.
http://gabeyg.myi.cc/grub.iso

Also, do I need device.map like in real HD booting?
Also, if MBR is the problem, is there a way to activate or merge two disk's MBR?
This site for Western Digital: http://www.wdc.com
WD - Service & Support Home
site for product infos: WD My Book Pro Edition II 1 TB Triple Interface Hard Drives ( WDG2TP10000 )
** I installed in the way the normal users install, so there is no LVM, no separte boot partition.

I also don't think it is kernel module problems (although I didn't try to put new module and re 'mkinitrd' make kernel.) (using ubuntu 7.04) , but I think it is grub's problem because it shows me Error 25 (using grub bootable CD) when pressed ubuntu.
**Also, when I booted from Grub cd, went to command line, I pressed find /boot/grub/stage1 and it shows me error 15. However, in ubuntu live cd, it showed me (hd1,0)! Also when I typed "root (hd0,0)"-my external drive when the bios is setted to boot my usb drive, it showed me error 25. So, I changed bios setting to boot from my internal drive and booted again from cd and typed 'root (hd1,0) and it shows me same error. However, when I do 'root (hd1,0)'-internal drive (XP) when bios is setted to boot from my usb drive, it does not show me any errors.