Most MP3 player are using NAND flash, mainly Samsung's or Hynix's, including some iPOD. This is classified as Flash MP3 player
Macrodriver and 1.8" HDD are used in larger MP3 player, larger in both size and capacity.. And they are named HDD MP3 player.
The advanced of Flash MP3 player is that it could have a longer play time, as Flash no machanical moving part while accessing the memory, such that less power will be used.
HDD MP3 player's advantage is that, HDD is still cost effective, when comparing the cost per byte... But as mentioned, it consume much power... that has shorter play time..