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I’m using two hard drives for conducting monthly backup for certain data in my PC.

However, I found out the folder the backup data inside my second hard drive has larger size on disk compare to the data in my first hard drive. This causes my second hard drive rapidly running out of space.

I did some research, and it could be the different file system in both of the hard drives might be causing this?

Can anyone verify if I change the second hard drive’s file system and allocation unit to match my first hard drive will it resolve the high size on disk issue?

First HDD

  • File System: NFTS
  • Allocation Unit: 4096 B

Second HDD

  • File System: ExFAT
  • Allocation Unit: 128 KB

Example of size on disk gap

Screenshot.

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    "Could the allocation unit size on my backup drive cause it to run of space when used to backup my main drive?" -- Assuming you're "backing up" by copying file by file, then yes. The difference in slack space between the two filesystems should average out to +124KB per file.
    – sawdust
    Mar 11, 2022 at 8:08
  • Oops. The difference in slack space between the two filesystems should average out to +62KB per file. IOW on average about half an allocation unit is wasted by each file.
    – sawdust
    Mar 11, 2022 at 10:40
  • If you can zip up groups of files before backing them up, you can solve this problem. As long as each zip file is larger than the allocation unit on your backup drive, you won't be wasting all that space.
    – Steve Rindsberg
    Mar 11, 2022 at 19:31

2 Answers 2

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Unless it's a pretty large volume and you're running into limitations with exFAT there shouldn't really be any reason to go for 128 kB allocation units.

With your current setup 1 kB file on your primary system is going to take about 4 kB at a minimum and on you backup disk 128 kB. If you do deal with a lot of smaller files this would explain the significant overhead.

Bringing them inline should reduce the on disk usage. In addition you might want to think about your chosen filesystem. Unless you have a particular need for it there shouldn't be a reason to use exFAT on your backup disk. Even most modern Linux distributions can at least read NTFS.

By design exFAT is more vulnerable to corruption than NTFS or other filesystems that offer journaling and e.g. redundancy of especially important information that's needed to use the system. As such using exFAT for your backup drive might not be the best idea.

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  • Thx for ur explaination i did another testing with a 1kb text file, first hdd only consume 1kb with 0 size on disk. However, second hdd show 1kb file size with 128kb size on disk. I guess changing the hdd file system and unit allocation will be solving my issue.
    – Axen Wong
    Mar 14, 2022 at 9:27
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Just look at the image you provided. On the right it says 41.9gb of space and right below that it says 41.9gb of files is using 65.2gb of hard drive.

The drive on the left is only using 42.4gb for the same files.

Therefore you are wasting 22.8gb of hard drive.

While an extra 22gb is a moderate amount even if you have a 128gb hard drive you should still have a lot of space.

So unless your using a 64gb hard drive or smaller I would not classify 22gb as "filling up your hard drive"

In short, by changing to a 4kb allocation unit you will get 22gb back. So if your expecting to gain 100gb in free space your out of luck, but if your expect 22gb back then your correct.

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  • Thx for ur explaination, bcs both hard disk will be fill with monthly backup data, hence it one of the hdd running out of space i have to purchase another hdd for the backup. However, i found the solution from ur answer. Thx
    – Axen Wong
    Mar 14, 2022 at 9:30

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