Data is written to NAND flash memory in units called Pages. Memory Blocks are made-up of many Pages. However, the memory, can only be erased in the larger Block units. If the data in some of the Pages of the Block are no longer needed( also called Stale or Invalid Pages), only the Pages with good data in that Block are read and re-written into another previously erased empty Block. Then the free pages left by not moving the Stale data are available for new data.

Thus, while an empty Page in a Block can be written to directly, it can not be overwritten without first erasing an entire Block of Pages. As the SSD is used(data written), data changes and the changed data is written to other free Pages in the Block or to new Blocks. At this point, the old(Stale)pages are marked as Invalid and can be reclaimed by erasing the entire Block of Pages. To do this, any still valid data on all of the other occupied(filled) Pages in the Block must be moved to another Block.

This process also consolidates Pages by moving and rewriting Pages from multiple Blocks to fill up fewer new Blocks. The old Blocks are then erased to provide storage space for new incoming data.