I'm very pleased that Arch is now running on MACCHIATObin. It's a very nice platform. Thanks to everyone involved.
I am running with uboot on an SD card and my root fs on a SATA drive. Here are the steps I followed to make this work.
1. Install a SATA drive.
2. Install ALARM as described on the ALARM website onto an SD card and boot from the SD card.
3. Follow the ALARM installation process a second time but target the SATA drive this time (probably /dev/sda).
4. Using serial, boot into the SD's uboot and update the boot_targets variable to have scsi0 first instead of last.
Now booting will use the uboot installed on your SD card, and it do the rest from the SATA. The SD card can be fairly small and slow, it's only used for uboot.