Question: Why Windows leaves some unused space at the end of some drives when creating partitions?

Those drives are MBR, not GPT. Their purpose is to store user data, not to boot operating system. Filesystem is NTFS.

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Is it any harm if I use gparted to use the full length of the disks?

Is there any advantage leaving an unallocated space at the end?

Answer: NTFS filesystems are allocated on disk in terms of “Clusters”. That is,a NTFS filesystem will never have a partial cluster at the end. Clusterscan be specified (by default or by the user) to have a number of differentsizes. The smaller the clusters are, the closer to 100% partition space thatcan be used. That said, the amount of unallocated space shown is trivial. I wouldn’t be at all surprised if no formatting/partitioning software writer would consider it worthwhile to avoid that unused space, but you’re welcome to shop around.