Setting Quotas for Disk Space Use
Disk quotas allow you to monitor and limit disk-space usage on NTFS-formatted drives and volumes—a handy feature on a multiuser system where a download-happy family member can easily fill up even the most gargantuan physical disk with digital music, videos, and pictures. Using quotas, you can set storage limits for each user and each volume. When you enable disk quotas, Windows tracks volume usage by file ownership. A file counts against a user’s quota if the security identifier (SID) of the file’s owner is the same as the user’s SID. For an in-depth explanation of SIDs and other security issues, see "Introducing Windows Security".
Disk quotas rely on the SID, not the user account name. In a workgroup environment—where ...