I get this error first:

Msg 102, Level 15, State 1, Line 28

Incorrect syntax near '>'.

With the syntax of yours.

The I fix it with this syntax:

(CAST(SUM(nUsed_Min) AS FLOAT(2)) / CAST(SUM(nSize) AS FLOAT(2)) * CAST(AVG(100)) >= 80)

Then i get the following error:

Msg 102, Level 15, State 1, Line 1

Incorrect syntax near 'CAST'.

Then I fit it with this syntax:

(CAST(SUM(nUsed_Min) AS FLOAT(2)) / CAST(SUM(nSize) AS FLOAT(2)) * CAST(AVG(100) AS NUMERIC(10, 2)) >= 80)

Then I get the following error:

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.

:S eehm