What is "Hi-MD"?
Introduced by Sony in January 2004, Hi-MD is a nearly complete revamping of the original Minidisc system. The most significant change is the introduction of Hi-MD media, which includes a new 1GB blank in the existing MD form-factor and a reformatting of existing MD media that doubles its capacity to 305MB. In addition, several important and long wished-for Minidisc capabilities have been incorporated into Hi-MD equipment.
What new capabilities does Hi-MD equipment provide vs. the existing Minidisc?
Hi-MD equipment can record audio to Hi-MD media in uncompressed PCM (16bits/44.1khz, i.e. CD format) as well as in Sony's ATRAC3plus encoding at 256kbps ("Hi-SP") and 64kbps ("Hi-LP").
Hi-MD recorders can be used directly as USB connected data drives on any PC supporting USB storage devices (no additional drivers needed).
Microphone and line input (i.e. analog source) recordings made on Hi-MD equipment can be uploaded at high-speed to Windows PCs via SonicStage (Sony's copyright-protecting audio transfer software).
What is Hi-MD media?
There are two Hi-MD media formats: a new 1GB magneto-optical medium based upon DWDD ("Domain Wall Displacement Detection") technology, and the original Minidisc magneto-optical medium (60', 74' and 80') reformatted to double its capacity. The Sony 1GB Hi-MD disk is the first commercial use of DWDD technology (as of January, 2004).
http://www.minidisc.org/hi-md_faq.html 
