Secondary storage, or storage in popular usage, differs from primary storage in that it is not directly accessible by the CPU.
The computer usually uses its input/output channels to access secondary storage and transfers desired data using intermediate area in primary storage.
Secondary storage does not lose the data when the device is powered down—it is non-volatile.
Per unit, it is typically also an order of magnitude less expensive than primary storage. Consequently, modern computer systems typically have an order of magnitude more secondary storage than primary storage and data is kept for a longer time there.
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