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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?

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    1) Windows only does what you tell it to do. If there is unallocated space, at the end of a drive, its because it was told to leave it there. – Ramhound Mar 22 '16 at 23:24
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    @Ramhound I have windows clean installs do that without asking. – Moab Mar 22 '16 at 23:24
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    @Ramhound Not true. I 'told' windows to allocate ALL space available. it still leaves something unallocated. – Azevedo Mar 22 '16 at 23:25
  • I have no idea how you created those partitions. I am just telling you the facts, if there is space left at the end of a drive, its because it was told to do that. – Ramhound Mar 22 '16 at 23:26
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    I created those partitions using Windows built-in "Disk Management" tool. I tried and tried. It keeps leaving some space at the end. On various HDD vendors and sizes. – Azevedo Mar 22 '16 at 23:28
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NTFS filesystems are allocated on disk in terms of "Clusters". That is, a NTFS filesystem will never have a partial cluster at the end. Clusters can be specified (by default or by the user) to have a number of different sizes. The smaller the clusters are, the closer to 100% partition space that can 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.

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Modern kernels tend to create partitions so that their boundaries be at positions where data blocks don't have to split during fetching operations. I am far from being a windows expert but I dare to state that if you did not want win to create like that, then the mentioned optimization algorithm led to this structure.

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Even though HDDs are addressable by LBA since decades, Microsoft still uses CHS for stupid stuff in the MBR even in the newest versions of Windows. That's why changing CHS translation in any way leads to a non-booting Windows. Or why sometimes partitioning gets completely garbled if you partition with Linux, then change a partition with Windows.

So when Windows creates partitions, it still tries to align to track boundaries. Especially at the end of the disk. Which is not only Microsoft's fault because there are some ancient BIOSes that would restrict access to incomplete CHS tracks at the end or even have an off-by-one in the track counter. Still, those should have died off long ago, and it usually works fine if you partition with Linux and do not touch the partitioning in Windows.

tl;dr: Legacy reasons.

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