What is the difference between secondary storage and disk?

Depending on the model you are using, it depends on what you mean.

Typically, most models have primary storage as the Hard Drive (or now, Solid State drive may be as well), a type of nonvolatile storage whose data does not disappear if it is unplugged or loses power.

Secondary storage usually refers to things such as CDs, DVDs, Floppy, Zip, and Tape drives. However in some textbooks it may just mean a second HDD/SSD.

Tertiary storage pretty strictly refers to CDs, DVDs, etc.