First, the Win32_DiskPartition class does not have a "DriveLetter" property, that query is going to fail every time.
Next, drive letters are Logical Disks, not physical ones. A single physical disk can contain multiple logical disks, each assigned to a Win32_DiskPartition instance through the Win32_LogicalDiskToPartition class.
Since WMI does not use a JOIN query, you have to rewrite it as an ASSOCIATORS OF query, like this:
ASSOCIATORS OF {Win32_LogicalDisk.DeviceId='E:'} WHERE ResultClass=Win32_DiskPartition
This query will get you the Win32_DiskPartition object for the specified drive letter.