A format for providing platform interoperability and native OS support for CD-RW and DVD+RW disks. The "MRW" or "CD-MRW" format enables files to be saved to RW disks as if they were hard disks (from any Save dialog or dragged and dropped). Mount Rainier supports the UDF file system and automatically formats a blank disk when it writes it for the first time. It allows the disk to be ejected before it is fully formatted by "de-icing" it, which saves the data necessary to continue the format later.

Mount Rainier also supports defect management, which masks bad sectors and writes the data into spares like a hard disk. In addition, it reduces block size from 64KB to 2KB for more efficient use of disk space. A driver can be added to legacy CD-ROM and CD-RW drives for read compatibility with MRW disks. See CD-RW.