Title says it all really, I've noticed this a few times in the past and again tonight after using the NAS. After 10 minutes the disk in use went to sleep but the large central power light stayed brightly lit instead of dimming as usual. Restarted the box and it slept "properly" after 10 mins. The NAS is configured as two separate disks but I'm sure this happened before in RAID1 mode. A software glitch perhaps?