disk space administration

I have a user with a 6GB drive partitioned into 3 areas. The second area (D:) has 1.99GB of total space. The user has only 18MB of data in this area so far. It has been working fine for many months. Last week he tried to copy a small file to the disk and gets error: Insufficient disk space….
Disk administrator shows 1.97GB free. What is happening here???