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I just bought a new external HDD (WD My Book 4 TB, it's my 3rd of the 4 TB variant).
As per usual, I formatted it using NTFS with 4KB cluster size because I'm using Windows 10, but then when I checked the drive's Properties it said that it has 4.89 GB of used space.

Drive properties:

enter image description here

I've had 3 WD My Book drives before this, and I'm pretty sure that this is the first time I encountered such a thing.
Then I ran fsutil fsinfo ntfsinfo command on cmd and this is the result:

NTFS Version      :                3.1
LFS Version       :                1.1
Total Sectors     :                7.814.031.359  (3,6 TB)
Total Clusters    :                  976.753.919  (3,6 TB)
Free Clusters     :                  976.698.434  (3,6 TB)
Total Reserved Clusters :              1.226.592  (4,7 GB)
Reserved For Storage Reserve :                 0  (0,0 KB)
Bytes Per Sector  :                512
Bytes Per Physical Sector :        4096
Bytes Per Cluster :                4096
Bytes Per FileRecord Segment    :  1024
Clusters Per FileRecord Segment :  0
Mft Valid Data Length :            256,00 KB
Mft Start Lcn  :                   0x0000000000000003
Mft2 Start Lcn :                   0x000000001d1c0a7f
Mft Zone Start :                   0x0000000000000040
Mft Zone End   :                   0x000000000000c820
MFT Zone Size  :                   199,88 MB
Max Device Trim Extent Count :     63
Max Device Trim Byte Count :       0xffffffff
Max Volume Trim Extent Count :     62
Max Volume Trim Byte Count :       0x40000000
Resource Manager Identifier :      399C28BD-BF92-11EB-95D1-448A5BD06D05

It says that it has Total Reserved Clusters of 4.7 GB and 200 MB of MFT, which is pretty much in line with the report of 4.89 used space on the drive Properties.
But why does it have such a big Reserved Clusters size? Is this normal?
So far, I have a total of 8 HDDs of various capacity (internal + external) + 1 SSD for the boot drive. I've checked and found out that all the other drives only have Total Reserved Clusters of mere megabytes, most of them have 5.8 MB and the biggest was the SSD with 22 MB of Reserved Clusters.

By the way, can somebody tell me what Reserved Clusters are for?

I've tried deleting the partition and reformatting the drive using EaseUS Partition Master (I used the Windows built-in drive formatter the first time around), but the result is the still the same.

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  • There is a difference between the "MFT size" and the "MFT zone size". Use Sysinternals ntfsinfo to see the different values.
    – Robert
    Commented May 28, 2021 at 13:29
  • @Robert Is it the one from this link? docs.microsoft.com/en-us/sysinternals/downloads/ntfsinfo It says "MFT size : 0 MB (0% of drive) MFT start cluster : 3 MFT zone clusters : 60352 - 111584 MFT zone size : 200 MB (0% of drive) MFT mirror start : 488376959"
    – LuckMan
    Commented May 28, 2021 at 13:33
  • Seems like someone also had this problem with a WD drive, although it's an internal one. what.thedailywtf.com/topic/27583/… And they have even much crazier Total Reserved Clusters size. At one point, it reached 595.1 GB.
    – LuckMan
    Commented May 28, 2021 at 14:52

1 Answer 1

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Microsoft gives the not-so helpful description in the article
NTFS_VOLUME_DATA_BUFFER structure (winioctl.h):

Reserved clusters are the free clusters reserved for later use by Windows.

It's not well-known when exactly they are reserved and for what. It's thought that they happen when:

  • Compression is enabled for part or all of the volume
  • Defragmentation of an inflated MFT liberated space that is kept for future use of the MFT.

There is no known sure solution, except of course for re-formatting the disk.

In the post Strange behaviour from new hard drive, for example, perhaps enabling/disabling compression did help.

My advice would be to not do anything - 4.7 GB lost from a total of 3.6 TB is really peanuts.

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  • "4.7 GB lost from a total of 3.6 TB is really peanuts." I agree. Maybe I'm just worried that it will get extremely larger, like in that post at one point the Reserved Clusters reached 595.1 GB. The drive actually came preformatted using exFAT, so I tried reformatting it to exFAT (the used space was reasonably only 6 MB), and then I reformatted it to NTFS again. At first, the used space was only 200 MB (for MFT), I confirmed it using both fsutil and Sysinternals ntfsinfo. HOWEVER, after that I decided to restart the PC, and voila, the 4.7 GB Reserved Clusters came back.
    – LuckMan
    Commented May 28, 2021 at 16:43
  • Unplugging and plugging back the drive (moments after formatting to NTFS) also triggered the return of the Reserved Clusters. I just wonder why it starts happening now. I had never experienced this before. Maybe it's Windows 10? My other 3 rather old WD My Book were all formatted on Windows 7.
    – LuckMan
    Commented May 28, 2021 at 16:52
  • Perhaps - NTFS on Windows 10 is a later version than on Windows 10 (only downwards compatible in format). There might exist some new feature that can be used only in the Windows 10 format.
    – harrymc
    Commented May 28, 2021 at 19:33
  • So, I just tried formatting it on Windows 7, and the problem persisted. So, either Windows 10's NTFS has "latched" something onto the disk that just can't be changed (probably unlikely), or maybe it's the new drive system? I also tried enabling compression on the drive and copied a 4 GB file, the Total Reserved Clusters was increased by 4 GB. Disabling compression freed up that 4 GB of space. If I had buckets of money, I would've tried shucking the drive just for the sake of curiosity.
    – LuckMan
    Commented May 29, 2021 at 3:13

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