Heat-assisted magnetic recording (HAMR) disk drive with disk having multiple continuous magnetic recording layers

A heat-assisted magnetic recording (HAMR) disk drive has a disk with at least two independent data layers (RL1 and RL2), each data layer storing an independent data stream. At a high laser power both RL1 and RL2 are heated to above their respective Curie temperatures and a first data stream is recorded in both RL1 and RL2. At a lower laser power only upper RL2 is heated to above its Curie temperature and a second data stream is recorded only in RL2. The data layers are separated by a nonmagnetic spacer layer (SL) that prevents lower RL1 from being heated to above its Curie temperature at low laser power. The first and second data streams are typically asynchronous. Recorded data is read back from both data streams simultaneously as a composite readback signal. A joint Viterbi detector detects the asynchronous data streams simultaneously from the composite readback signal.