Karim, I overlooked at the following message:

Msg 147, Level 15, State 1, Line 28

An aggregate may not appear in the WHERE clause unless it is in a subquery contained in a HAVING clause or a select list, and the column being aggregated is an outer reference.

This clearly shows the wrong implementation while using aggregate function. The code should be like this:

Select TOP 10 nDeviceId,

sDisplayName,

sgroupname,

sdescription,

dpolltime,

MIN(nused_min),

MAX(nused_max),

SUM(nsize),

(CAST(SUM(nused_min) AS FLOAT(2)) / CAST(SUM(nsize) AS FLOAT(2)) * CAST(AVG(100) AS NUMERIC(10, 2))) AS Inuse

from statisticalDisk

group by nDeviceId,

sDisplayName,

sgroupname,

sdescription, dpolltime

having (CAST(SUM(nused_min) AS FLOAT(2)) / CAST(SUM(nsize) AS FLOAT(2)) * CAST(AVG(100) AS NUMERIC(10, 2))) >= 80

Check this out and let me now if it works. Look out for using aggregate functions usage in BOL.