Ask for help of grub

[cnboot]

Hi,I am the Linux user,come from China,I am writing to you for help.I wanna 
install the Arch Linux into my USB device. I have some problems in grub.  I 
divide my USB device to two partitions(sdb1 and sdb2,sdb1 is ext4 filesystem 
and the sdb2 is fat32,they are both primary partition). Then I install the Arch 
Linux to the sdb1 by "pacstrap" command ,and mount the sdb2 on /mnt,I run the 
"grub-install --boot-directory=/mnt /dev/sdb" to install the grub. And it can 
boot. But as you know,the Windows system can not recognize the two partitions 
at the same time ,it can only read the sdb1,so i change the sdb1 to sdb2 and 
sdb2 to sdb1 so that I can read the another partition which has the grub.cfg 
file ,but I find the USB device can't boot normally, it falls to "grub rescue " 
mode with the wrong message "unknown filesystem":-( And i run "set",it tells me 
the root=(hd0,msdos2) which stores the grub modules and grub.cfg.But it still 
can't boot ,it is obvious that the grub doesn't load the modules and grub.cfg.I 
don't know why:(.I remember that i have done the same thing some weeks ago and 
it worked well.But now it can't work.And now i use "bootice" which is a windows 
partition tool to install the grub,this time no matter how i change the sdb1 or 
sdb2,it can also read the two partitions ,but when run the "normal",it tells me 
that "grub_real_boot_time" not found :( Oh i really don't know how to deal with 
it.
I hope you can help me :)