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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?

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Secondary is, well, "secondary". If "main" storage is "primary", then disk or whatever is "secondary". Secondary storage, in theory, may or may not be "offline" (I mean printouts are technically "secondary storage"), but generally one would expect the term to be applied to online storage. –  Daniel R Hicks Jan 10 '13 at 19:23
    
The term originated in the Dark Ages of computing when a few KB of RAM was a lot. Data and programs would be moved in and out of main storage as needed, to leave enough space in main storage to actually do something. Well before "virtual memory" was developed, programmers viewed the secondary storage as an extension of main store. –  Daniel R Hicks Jan 10 '13 at 19:27
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Wikipedia to the rescue

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:

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Thanks a lot! Actually I had read that article on Wikipedia before you answered my question, but I didn't understand all, and therefore was in need of a more exact explanation. –  petajamaja Jan 9 '13 at 21:07
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