The best replacement for free space is unallocated.

The best alternatives to free space are rated as following:
★★★★★  unallocated
★★★★  unallocated space
★★★  slack space

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Speaking about free space, one probably shouldn't forget about c drive.

Other search terms related to free space and their ratings are:
★★★★★ c drive
You have in mind a companion for free space and it is not yet here? Propose!



Google Trends

Scale is based on the average worldwide traffic of free space in all years. Click on graph for more.
free space
unallocated
unallocated space