Mount Rainier (MRW) is a format for writable optical discs which provides the packet writing and defect management.
Rainier allows write access to a disc within seconds after insertion and spin-up, even while a background formatting sequence is taking place.
It is even possible to read (but not write) MRW disks without an MRW-compatible drive; A "remapper" device driver is needed, an example of which is EasyWrite Reader for Windows.
Operating systems that don't support MRW natively (notably Windows XP and prior versions) need third-party software to read and write MRW-formatted discs, and these tend to be the same packet writing utilities which allow native UDF filesystems to be written to optical media.
[4] The EasyWrite logo is the marketing symbol created by Philips for CD drives that are Mount Rainier compatible.