You can reinstall from scratch if you first revert to manufacturer status.
http://Arch Linux ARM.com/index.php5/Talk:Install_on_Pogoplug_Pro
To do this you ape the instructions in the install guide but substitute the revert script for the install script.
You can't revert unless you are logged into a working Arch Linux ARM Linux install.
The revert script does not destroy the Arch Linux ARM Linux install on the drive, it just modifies the Popoplug back to factory state so it does not boot from your PlupApps install.
If you install Arch Linux ARM to a different disk and then swap out the new disk with your original, your original will boot. (This saved me once from trouble when I borked Pacman). Do not try to reinstall Arch Linux ARM onto a disk that already has it... you must first erase the contents of the drive or the script will fail due from files already existing.
I actually think it would be useful to have a semi-install script that modifies the Plug to boot from a Arch Linux ARM drive but does not reinstall anything. That way you can keep a little USB disk with a fresh install to act as a recovery disk for various tasks.
Anyway, to recap. Revert with the following sequence:
$this->bbcode_second_pass_code('', '#~ cd /tmp
#~ wget http://Arch Linux ARM.com/os/2011.04/oxnas-revert.sh
#~ chmod 755 oxnas-revert.sh
#~ ./oxnas-revert.sh')
I have done this three times. The last time it was with a 2011.06 install but the 2011.04 revert script worked fine for me.