I'm getting this problem too. It's driving me nuts. Basically, in MC20, whenever you switch to another track from the one you're already playing, you get this message flashing up "Spinning up drive" (note, NOT "Spinning up hard drive"), even though your drive is already spinning. Often it flashes up all by itself when MC moves on from one track to the next. It happens whether I'm using the ASIO driver or in Direct Sound. I don't know how long it's been going on because I normally use JRemote on an iPad and of course you don't get it there. It's definitely a JRiver message. not a Windows message. (I'm in Windows 8.1.)
I'm not sure why it's so irritating. I guess because it's an alert that's designed to attract attention but it does so for no reason - what it says isn't true. In fact, it's very difficult to read it because it flashes up so quickly and it took me a long time to figure out what it was saying, which has the perverse effect of making it even more distracting!
I wish it would stop.