DISK MIGHT NOT BE SPUN DOWN PROPERLY ...

[Gerard Robin <g.robin3@free.fr>]

Hello,
when I stop my box (sid amd64) with: 
sudo halt (or shutdown -h now) 
I get this message:

DISK MIGHT NOT BE SPUN DOWN PROPERLY UPDATE SHUTDOWN UTILITY

Why do I get this message and what does it mean ?

tia
--
Gérard