What is the difference between secondary and offline memory? Both are not directly accessible by CPU, non-volatile,and removable. A flash memory is said to be both secondary and offline. Maybe one of those storage types is subpart of another?
You can look at it as being a subpart, but its more of a heirarchy.
Mass storage devices are considered secondary storage - that includes hard drives, flash drives and other devices.
Offline storage is storage which is swapped out by a user - that includes hot swap hard drives and flash drives (if being swapped out frequently by a human operator), CDs, blu-ray disks
To my knowledge, the terms secondary and offline storage aren't typically used to describe the device, but instead how the devices are used. The terms are more academic than technical.
See the following image for more details: