The different SD card bus speeds are described here:
https://www.sdcard.org/developers/overview/bus_speed/Note that for UHS-I there are actually multiple different bus speed modes including SDR50 (50 MB/s) and SDR104 (104 MB/s). For UHS-II the bus speed is 156MB/s Full Duplex (read/write) or 312MB/s Half Duplex (read or write only).
The bus speed is just the maximum theoretical speed data could be transferred over the interface between the card and device. In most cases the read and write speed of the memory on the card is significantly lower than the bus speed. And the memory in most cards has write speeds lower than the read speed.
Assuming the device reading or writing to the SD card supports SDR104 for UHS-I then it should be able to read and write to cards that support SDR104 at the maximum speed the card supports. For example, some of the early SanDisk UHS-II cards supported 312MB/s transfer speeds over the UHS-II bus, but only supported SDR50 over the UHS-I bus. So if these cards were inserted in a UHS-I card reader/writer, they would max out at 50 MB/s. More on that here:
http://www.cameramemoryspeed.com/review ... mory-card/The same is not true for Transcend UHS-II cards as these cards support SDR104 over UHS-I and max out at just below 104 MB/s when inserted in a UHS-I card reader:
http://www.cameramemoryspeed.com/review ... mory-card/I'm not aware of situations where UHS-I cards that support SDR104 operate slower in UHS-II card readers/writers. My understanding is that this would only be the case if the card reader/writer in the device did not support SDR104 for UHS-I and only supported SDR50. But I believe most (if not all) UHS-II card readers support SDR104.
Although the specs for the HyperDeck Mini's SD card interface are not given, this information is provided for the Blackmagic Video Assist 4K which does support SDR104. It would surprise me if the HyperDeck Mini did not support SD104 for UHS-I.
Because consider that the bitrate for ProRes 422 HQ at 1080p60 (the maximum HD recording mode for the HyperDeck Mini) is 440 Mb/s or 55 MB/s. This is more than the maximum bus speed for SDR50 (50 MB/s), which means that no UHS-I cards would be usable for HD recording in this mode. But there are some approved UHS-I cards for HD recording.
If you consider that the bitrate for ProRes 422 HQ at 2160p30 (the maximum Ultra HD recording mode) is 884 Mb/s or 110.5 MB/s, it makes sense that UHD recording requires UHS-II cards since the maximum bus speed of UHS-I is 104 MB/s. In principle, some of the lower ProRes levels like ProRes 422 LT at 2160p30 (51.25 MB/s) should be possible using UHS-I cards since this is within the SDR104 bus spec.