Windows server has great deduplication functionality allowing for increased usable capacity on non-operating system volumes, by performing block level deduplication to increase storage efficiency. This is particularly useful for storing data such as backups, which often involves minimal changing data, so a prime candidate for effective deduplication.
The problem often encountered with deduplication volumes is that when files are deleted, the space is not immediately reclaimed by the operating system. A "garbage collection" process needs to be run in order for the deduplication engine to reclaim this space.
Thankfully, this is incredibly easy to do and can be done from a powershell command prompt - here's the command you need to run;
start-dedupjob -type GarbageCollection -full -path e: -FastStart $true
