Some harddrives perform thermal realignment of the heads every once in a while (different metals expand differently as they get warmer).
Also, IIRC some SCSI controllers perform some polling (similar to the floppy click) to look for new devices on the bus every now and then, because of the nature of scsi (e.g. you have an external CD-ROM drive hooked up, but switched off. Then you need to use it, and simply switch it on, without any reboots. The SCSI controller finds it during its polls, and you can mount CD0:)
-Paul
