errors in event log

Hi,

i have a compaq ML370 Server in which Windows NT4 workstation is loaded

last week i restarted the machine once after that its throwing an error in event viwer-system log

Event id : 15
source:atapi
Type : error
Category : None
Description:
The device, \Device\ScsiPort0, is not ready for accees yet
—–
this error repeats in every 20 seconds

why its happening so, pls help

Thanks