Error 17: cannot mount selected partition

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https://bbs.archlinux.org – I tried to install Arch Linux onto an external hard drive, I made a root partition (ext3), and a little swap space. I flagged the root partition "boot" and I installed the grub loader onto the dev/sdb/ (not the root partition). Looked like a successful installation, but when I restarted grub was loading and quit part way with the Error 17: cannot mount selected partition... the line above that stated "/boot/vmlinuz26 root=/dev/sdb1 ro. sdb1 is my root partition. ()