What are secondary memory devices?
By definition of the term, primary memory is that memory which the processor can access directly. It is also called RAM. It is volatile (empties when electricity is switched off).
In contrast, secondary memory is that which the processor cannot access directly; its content must be loaded into RAM first. Also, such memory remains persistent when electricity is switched off.
Not by definition but those devices are slower ones but can store much more data.
These devices are hard disks, flash drives, optical disks and so on. Magnetic tapes were one of the first secondary memory devices.