Do you have a dual-boot OS or any local clones that could "accidentally" be booted into? Sometimes double-checking the target startup disc in those cases makes all the difference.
Also, do you have the original card installed in addition to the GTX 680, or are you solely using the GTX 680?
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Thanks - likely going to be holding out until the 10.9.1 or 10.9.2 availability before I install on the MacPro. Hopefully by then any kinks are worked out. Only issue with 10.9 on MacBookPro so far is all Apple Mail related, but nothing so major that I'm not using it.