If I install Windows onto a disk which has multiple partitions (eg. C:\ and D:\), and set the target as C:\, a few of the folders will be created on the D:\ partition. Windows will work perfectly well, but the D:\ will sometimes be used when installing new apps. For example, installing Visual Studio 2008 Express will result in a temporary folder being created on the D:\ partition.

However, if I only have a single partition when Windows is installed and then create a secondary partition afterwards, Windows does not use the second partition and temp folders are not created on the second partition during installations either.

I have checked the list of environment variables and they all point to the C:\ partition.

Why does this happen ?

Could it just be poorly coded installers that for whatever reason result in some files ending up on the secondary partition ?

I believe that certain installers choose the partition or disk with the most free space available.