When you compress a drive, some files are exempted, but you can force it, and then it's your problem
On the drive property sheet for an NTFS volume, there is a checkbox called "Compress drive to save disk space." If you check that box, the shell marks the drive as "compress all newly-created files" and also goes through and compresses all the existing files on the drive. Well, almost all the file. Some files are exempted by default. Examples of...