I'm currently using a SanDisk SDDR-B531-GN6NN micro SD reader. The micro SD card I'm using with it is a SanDisk 32GB "Max Endurance". Had no issue installing unraid on the SD card or registering my licence key to the card reader's GUID.
I like this SD card reader because of its a small form factor, barely stick past the case's recess for the MB i/o shield. It's small enough that it's even suitable for being installed internally using an internal USB port if you're using a server mobo or a USB header to USB A adapter like this or this (I haven't bought or used either of the adaptors linked, they're just examples not recommendations).