More Disk Space Required to Copy or Move Compressed Files

This article was previously published under Q137239

Windows NT does not copy or move compressed files between two compressed NTFS partitions in a compressed state. When you open a compressed file, the NTFS file system driver (NTFS.SYS) uncompresses the file as it is read. When you attempt to copy or move a file to another compressed NTFS partition, an attempt is made to allocate enough disk space on the destination partition to hold the file in an uncompressed state.

If the destination partition does not have enough disk space to accommodate the file in an uncompressed state, the copy or move operation fails.

If there is enough disk space to move or copy the file uncompressed, Windows NT compresses the file as it is written to the compressed NTFS partition. The unused portion of the pre-allocated, uncompressed disk space for the file is then released as available disk space.