Well, I know in the past, gear was super fussy about what cards they would or wouldn't work with, and I remember this from my days of using my Roland MS-1 and my Alesis QS-8 keyboards. Seems Sandisk was the brand to go with unless you wanted bad results. Not knocking Sandisk, I like their stuff, a lot, and still use their brand.
I would think in this day and age, with memory cards and sticks becoming a commodity-type item, nobody can afford to be brand specific as long as the tech specs are met. I will say this though, DO be aware of big capacity cheap cards, because some use a compression algorithm that may double space(say a 4Gig card to an 8Gig), but they tend to have a high fail rate. You don't buy these from mainstream stores, you gotta go to questionable locations to get that junk.
I'm wondering if there is a voltage control issue in the interface, or else perhaps the contacts are just getting dirty? But I see you having 2 of these units, so why would one get dirty and the other wouldn't? I mean, you're using them at the same time and transporting them the same way...
This is definately a support issue. Call the main number and ask to speak to service. You might need to call back a few times to get through.
I do agree, it shouldn't be so fussy. If the cards meet the tech specs, it shouldn't matter after that.