My understanding is that all of the timing-critical events involved in writing to the card are handled by the SDI peripheral independently. So if it's in the middle of transmitting a byte to the card, it will continue with that operation right through any interrupt that occurs. And remember that the master provides the clock signal for transfers in both directions, so even if one clock phase was a little longer than normal, it shouldn't matter. So I don't think it's necessary to turn off interrupts during serial traffic so long as you're letting the hardware peripheral do it, and are not using software serial of some kind.
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