You also forgot to tell us which S5 model you have. There are six or more Korean variants. So don't forget to include your specific model in your search string.
My question should be more precise in that I'm looking to get a 32GB S5 (any model that's available) to use in the USA with Straight Talk. I mentioned the Korean versions only because I don't see reasonably priced 32GB S5's from US (GSM) carriers. My Straight Talk SIM is of the AT&T variety.
The additional information is appreciated. There are a lot of knowledgeable and helpful people in this forum. But it can get tiresome to be inundated with questions from people who clearly made little effort to search before posting. I don't have any interest in facilitating indolence. I'd rather spend my limited time helping someone who made an effort in the first place.
The information you are looking for is hard to come by. Samsung would prefer that you buy a new phone if you switch carriers. And carriers don't want to facilitate churn. So there are a lot of artificial barriers and restrictions between models to discourage grey market use.
To properly answer your question would take an hour of my time digging through six or ten sets of Samsung specs and doing several searches to see what ATT, Straight Talk and the FCC are doing with spectrum allocations. And if you think that I have that much spare time or interest.. you Sir, are an optimist.
I took a stab at it for you anyway and checked just one of the Korean models. The L variant is essentially compatible on 2G /3G (voice and low speed data). But LTE is another story. The Korean variant covers about two thirds of the data bands that ATT uses. But before we even begin to discuss the impact of that, looking at what Straight talk actually uses shows us that it's a non-starter.. it doesn't support any of the Straight Talk data allocations.
As for the other half dozen Korean models.. who knows? Esoteric information. Some members will understand the nuances of data sheets and spectrum usage. Precious few will have the information at hand though. It's an unusual situation that few will have researched in advance. So you have a barrier there, convincing an expert to spend an hour checking tedious facts for you. You should make an effort to bridge the gap. If you put in the legwork, narrow down the options and come back with informed questions, a lot more people will be inclined to volunteer their time to help you.
Yes, that does sound like work. But it is your question after all, isn't it?