You may view your hard drive in Disk Management and see unallocated space. The space will be on the hard drive, but Windows cannot use it. It is like having a spare room in your house but locked off.
MUO on MSN
I cleared space on my SSD past the 80 percent mark and my write speeds immediately recovered
I let my SSD hit 90 percent full before I realized how badly it was choking my write speeds.
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