There are a couple of reported configurations where you might find that your machine does not recognise your BBC micro:bit device. 

Firstly, make sure  check you have the correct USB data lead.

Windows 10

If the micro:bit is not recognised or it is just listed as a FAT drive and you are unable to flash code to it, you should check on another device that you are running the latest firmware.

If you are using the Windows App, please refer to the Javascript Blocks Editor help for more information

Macbook Air (OSX Sierra)

If the micro:bit is not recognised, please update your machine if you are able. The micro:bit is reported to work correctly using High Sierra

Thanks to @ForrisHilier and Ari Keränen for identifying the respective resolutions