I don't know Macs well, but would be VERY surprised if they don't have some Hyperterminal-like program (HLP).
That could be "fired up", connected to the Arduino (and the Arduino connected to the Mac) and then the HLP would put up on the Mac screen the stuff coming from the Arduino. From time to time, the human could save what had been sent to the Mac since the last save.
Of course, this entails a human being at the keyboard, but if that's not impossible, it saves you a bunch of issues.
Some Mac person: Please jump in with the name of the Mac equivalent of Hyperterminal?
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Another issue: There are various ways to connect two computers. If you can endure a "crude" solution, you will have less hassle. By crude, I'm meaning a non-handshaked solution in which the Arduino "talks" without taking any notice of whether anyone is listening.