It looks like mkinitcpio.conf till mention the "raid" hook whereas it looks like it has been superseded by "mdadm" hook.... Indeed :

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Generating initial ramdisk, using mkinitcpio. Please wait... ==> Building image "default" ==> Running command: /sbin/mkinitcpio -k 2.6.32-ARCH -c /etc/mkinitcpio.conf -g /boot/kernel26.img :: Begin build :: Parsing hook [base] :: Parsing hook [udev] :: Parsing hook [autodetect] :: Parsing hook [pata] :: Parsing hook [scsi] :: Parsing hook [sata] FATAL: Hook 'raid' can not be found.

But replacing "raid" by "mdadm" leads to : # /sbin/mkinitcpio -k 2.6.32-ARCH -c /etc/mkinitcpio.conf -g /boot/kernel26.img :: Begin build :: Parsing hook [base] :: Parsing hook [udev] :: Parsing hook [autodetect] :: Parsing hook [pata] :: Parsing hook [scsi] :: Parsing hook [sata] :: Parsing hook [mdadm] Custom /etc/mdadm.conf file will be used in initramfs for assembling arrays. :: Parsing hook [keymap] :: Parsing hook [encrypt] :: Parsing hook [lvm2] :: Parsing hook [resume] :: Parsing hook [filesystems] :: Generating module dependencies :: Generating image '/boot/kernel26.img'...SUCCESS -Mathieu