I recently bought a 8TB HDD to replace my 1TB drive for storing my backups. However after moving the files to the new drive I noticed they take up a substantially larger amount of space on the disk, even though the true file size is the same. Both drives are formatted to be exFAT so I am not sure why the reported size is so different. Why is substantially more space being used on the new drive?

File size comparison (Left is old, right is new)

  • It's probably a different cluster size; might be worthwhile to use a more resilient filesystem if you're storing your backups on those drives. – Seth May 3 '19 at 6:08

The default cluster size for exFAT is extremely large. According to this answer it's like this

Volume size   Cluster size
  500 GiB          128 KiB
 1000 GiB          256 KiB
 1500 GiB          512 KiB
 2000 GiB          512 KiB
 3000 GiB            1 MiB
 4000 GiB            1 MiB
 5000 GiB            2 MiB
 8000 GiB            2 MiB
10000 GiB            4 MiB
20000 GiB            8 MiB

Your files are 753674490827/726245 ≈ 0.9897 MB on average, so on the 1 TB drive with 256 KB cluster, about half of the files take 4 clusters (and waste almost nothing) and roughly the remaining half need 5 clusters (and waste 1 cluster)

The average wastage is 0.5 clusters = 128 KB per file or 12.5%. The total expected size on disk is 753674490827 × 112.5% ≈ 789.7 GB which is extremely close to what's in your screenshot. However on the 8 TB disk, the clusters are 2 MB, so you're wasting ~1 MB per file, which is why the new size on disk is almost twice the total size and the previous size on disk


It's also the same reason as this question Why is there such a big difference between "Size" and "Size on disk"?

On NTFS there are many other reasons. If you're interested read Why does "Size on disk" vary when "Size" does not with the same set of files?

In short, exFAT is bad for storing small files. Either use NTFS, or reformat exFAT with a smaller cluster size (4 KB just like almost everyone else including NTFS)


Phuclv is right. If you are using ExFAT on a 8TB disk your cluster size will be 8MiB which means that any file will take at least 8MB of disk space, even if it's only a small txt file.

For a drive that big I would recommend ReFS (if you are using Windows 10 or 2012/2016 server) with a 4KB cluster size:

ReFS Cluster Size

or if you are looking for an even smaller cluster size go for NTFS:

NTFS Cluster Size

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