I thought PLX OxNAS is no longer supported, is it? If no, then perhaps you can try
Debian ARM (with
Bodhi's u-boot --
un-official released of newer version uboot.2013.10-tld-5.ox820.bodhi.tar) or may be
OpenWRT PLX OxNAS. If you intend to use the later, please do yourself a big favor to read more about it, i.e.
no rootfs in builds (you probably have to build your own from
OpenWRT source),
problem booting OpenWRT OxNAS from external storage with a stock firmware still intact on internal storage, etc. AFAICT, the later requires building from source with some tweaks in reconfiguring the Linux kernel source tree, i.e. including drivers for SATA, filesystems, USB, etc., in order to boot OpenWRT OxNAS from external storage while the stock firmware is still intact in the internal storage. AFAICT, your best chance is to go with Debian ARM.