[EndlessWaves]

Actually it's μ that's the symbol for Micro, that's the greek letter mu. The Latin letter u is occasionally used as not all keyboards have a mu key, especially outside Europe. It's also often simply abbreviated to m or M.

EDIT: On topic, 'safe removal' is often to prevent a device being removed as data is written to it. I'm not that familiar with the SD card standard but if the Kobo never writes to the SD card (stores annotations etc. on the main memory) then it may be designed to be pulled out at any time.