Thanks for made test (i don`t found old SD cards at home for check it)
As i see all SD card types work
SD cards work
SD HC cards work
SD XL cards should work if format to Fat32, but need test, possible i enable exFat support
Read/Write speed not big, but this limits of SD card SPI bus mode and processor.
In future i try
revert SPI speed slowdown to 36MHz
add save s1p and s2p data files as *.s1p or *.s2p (need read more about this format, it just text but possible variants)
I think this a good extend NanoVNA capabilities.
