解决了。。。没文化真可怕!
天杀的 简体中文 wiki 害人不浅。。。
解决方案:
Note: Arch Linux installers prior to the 2011.08.19 release included an option to install the bootloader to either a partition or the MBR. This option has been removed and so it is no longer possible to install the bootloader to a partition using AIF (see FS#25726). If you wish to install the bootloader to a partition boot record, simply exit AIF without invoking the "Install bootloader" step and install the bootloader manually.