There's probably a regulatory issue with unknown SD cards. Safety regulations require spacing between internal electrical circuitry and your finger. If you use an unknown SD card, the spacing between the electrical circuitry within the card may not be large enough to meet the regulatory requirements. It actually makes sense in terms of regulations. You make standard rules on spacing, voltages, etc. that apply to everything, and you sometimes apply it to something where it doesn't really make that much difference if you really think about it. It's an "err on the side of caution" type of thing.
As for "real" safety, the voltages inside the machine are probably not high enough to be very dangerous except in really unusual situations, but it could happen. Me, I use whatever SD card I happen to have handy.
Respironics probably has to say that for regulatory reasons. They probably also don't want to have to deal with someone else's SD card problems, either. Generalized greed in terms of wanting a monopoly on replacement parts probably plays a part, too.
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