Thanks all for your comments - I have the newest MacBookPro, with two Thunderbolt and two USB ports. I bought an external hard drive that has both hookups. I do not have a network set-up, everything is hard wired.
I downloaded a free trial version of both software programs, and I must say that Pure Music sounded significantly better in my system, and I have pretty much decided on it already on just the third day of the trials. Quite a bit bigger soundstage, and better imaging are the biggest differences. I also had some issues with the library portion of the Audirvana software.
I am by no means done experimenting yet, but so far Pure Music seems quite a bit better.