Booting from OWC drive enclosures

My Mac won’t boot up a Snow Leopard 10.6.8 system that I cloned onto my OWC Mercury Elite mini drive enclosure using either an eSATA or FW800 cable, but starts fine if connected via USB 2.0. In comparison, I can boot up an identical Snow Leopard system from an OWC Mercury On-The-Go-Pro enclosure that’s connected via FW800. Perhaps there’s a limitation in the Oxford 944SE chipset that only allows the Mercury Elite to boot up via USB 2.0.