Eh...
The MDD disks generally come in 76 and 80 minute varieties...
The Hi MD disks are the Gigabyte disks used in the newer MiniDisc machines...
It's not so much what the disc is like as much as what your player is...
If a player is newer and handles the Gigabyte Hi MD discs / they will generally read and play the MDD disks...
But, it doesn't go the other way around...
If you're talking about recording with a multi-channel recorder, I don't believe those units will format or use a Hi MD Disk...
Hope this helps...