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-- "One nuisance is that the SD card has to be formatted with ext4 file system"
That seems rather odd, largely because it makes it incompatible with out-of-the box Windows 10, both for reading and writing - and that makes it odd as a decision at the design phase. It is even more odd when you consider that most Linux distributions support read/write/format of FAT12/16/32 in the standard releases.
The manual explicitly states, for the DAR-101 -- "A standard '2GB' SD card Must be formatted into FAT 16 and all the SDHC card MUST be for-matted into FAT 32."
There are some recording devices out there that require ext4 - eg, the Epiphan Pearl Mini, for example (which is video capable and likely massively 'too much' for just audio recording), so there are some devices out there. Possible confusion with a different device?
I am curious in your experience on this, as I have been looking for a portable device that can handle portable recording using external mics, and the DAR-101 mostly fills the bill very nicely. But having an ext4 format requirement is likely a negative point for a device that hits many positive points.