How much internal storage space should be available before installing Windows 10 via the Insider program?
The process should only need around a gigabyte of space, but you should try to make sure you have ~3GB free.
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1Just in case someone might find this helpful, I lost about 1GB of space after the update, mostly due to default apps that you download from the store after installing Windows 10. The amount of space used by system files remained about the same at 2.5GB – Jake Feb 6 '16 at 17:32
