TSQL divide by zero encountered despite no columns containing 0

Question

I've been trying to understand why I get a "divide by zero encountered" (Msg 8134) with my SQL query, but I must be missing something. I would like like to know the why for the specific case below, I am not looking for NULLIF, CASE WHEN... or similar as I already know about them (and can of course use them in a situation as the one below).

I have an SQL statement with a computed column similar to

SELECT
    TotalSize,
    FreeSpace,
    (FreeSpace / TotalSize * 100)
FROM
    tblComputer
...[ couple of joins ]...
WHERE
    SomeCondition = SomeValue

Running this statement errors with the above mentioned error messages, which, in itself, is not the problem - obviously TotalSize might well be 0 and therefore cause the error.

Now what I don't understand is that I do not have any rows where the TotalSize column is 0 when I comment the computed column out, I double checked that this isn't the case.

Then I thought that for some reason the column computation would be performed on the whole result set before actually filtering with the conditions of the where clause, but this a) wouldn't make sense imho and b) when trying to reproduce the error with a test set-up everything works fine (see below):

INSERT INTO tblComputer (ComputerName, IsServer) VALUES ('PC0001',1)
INSERT INTO tblComputer (ComputerName, IsServer) VALUES ('PC0002',1)
INSERT INTO tblComputer (ComputerName, IsServer) VALUES ('PC0003',1)
INSERT INTO tblComputer (ComputerName, IsServer) VALUES ('PC0004',0)
INSERT INTO tblComputer (ComputerName, IsServer) VALUES ('PC0005',1)
INSERT INTO tblComputer (ComputerName, IsServer) VALUES ('PC0006',0)
INSERT INTO tblComputer (ComputerName, IsServer) VALUES ('PC0007',1)

INSERT INTO tblHDD (ComputerID, TotalSize, FreeSpace) VALUES (1,100,21)
INSERT INTO tblHDD (ComputerID, TotalSize, FreeSpace) VALUES (2,100,10)
INSERT INTO tblHDD (ComputerID, TotalSize, FreeSpace) VALUES (3,100,55)
INSERT INTO tblHDD (ComputerID, TotalSize, FreeSpace) VALUES (4,0,10)
INSERT INTO tblHDD (ComputerID, TotalSize, FreeSpace) VALUES (5,100,23)
INSERT INTO tblHDD (ComputerID, TotalSize, FreeSpace) VALUES (6,100,18)
INSERT INTO tblHDD (ComputerID, TotalSize, FreeSpace) VALUES (7,100,11)

-- This statement does not throw an error as apparently the row for ComputerID 4 
-- is filtered out before computing the (FreeSpace / TotalSize * 100)
SELECT 
TotalSize,
FreeSpace,
(FreeSpace / TotalSize * 100)
FROM 
tblComputer
JOIN
tblHDD ON
tblComputer.ID = tblHDD.ComputerID
WHERE
IsServer = 1

I am quite stumped and would like to know what the reason is.

Any ideas or pointers into the right direction are very welcome, thanks in advance

Update

Thank you so far for your input, but unfortunately I seem not to be getting closer to the root of the problem. I managed to strip the statement down a little bit and now have the case that I can execute it without errors if one JOIN is removed (I would need it for additional columns in the output which I temporarily removed).

I do not understand, why using the JOIN leads to the error, shouldn't a standard INNER JOIN always either return the same number of rows or less, but never more?

Working code

SELECT 
TotalSize,
FreeSpace
((FreeSpace / TotalSize) * 100)
FROM 
MyTable1
INNER JOIN 
MyTable2 ON
MyTable1.ID = MyTable2.Table1ID
WHERE 
SomeCondition

Error causing code

SELECT 
TotalSize,
FreeSpace
((FreeSpace / TotalSize) * 100)
FROM 
MyTable1
INNER JOIN 
MyTable2 ON
MyTable1.ID = MyTable2.Table1ID
-- This JOIN causes "divide by zero encountered" error
INNER JOIN 
MyTable3 ON
MyTable2.ID = MyTable3.Table2ID
WHERE 
SomeCondition

I also tried my luck using a cursor and looping over the result row by row, but in that case no error occurred (no matter, which of the two statements above I tried).

Sorry for the messy code indentation, somehow the correct formatting doesn't seem to be applied.

G.