purchased the SanDisk 256GB High Endurance Video microSDXC Card. Out of the box the K3 DVR could not read my Sd card. DVR faild to come on and no lights on the remote control. Points to non-compatible SD card. Re-formatted the SD card to Fat32 on my Mac. Same results. Re-formatted the SD card to Fat32 on a PC and the DVR was able to read the card and function properly. But the Mac was not able to read the card so I could update the firm ware. I Re-formatted the SD card in the DVR and now the Mac can read and write to the SD card. I am now able to update firmware using my Mac. One additional note is the SD card has 100mbps read speed and only 40 mbps write speed. It still works in the DVR but I am limited to High bitrate. Maximum bitrate will not work. I assume If I get a new SD card with better write speeds I should be able to use Maximum.