Windows - No Disk error

Some users of my app get a "Windows - no disk" error when one of my windows
opens up

I can't duplicate it, but I suspect its related to this

for Count:=3 to 3 do
if DiskSize(Count)>0 then begin
end;

Perhaps if the user has removed a hard drive "improperly"- this DiskSize()
routine gives an error?

If this is correct, is there a safe way to see if a disk is mounted, without
generating an error?