Same folder on two external SSD hard drives, both formatted in ExFat, not taking the same amount of space

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rom1

PCHF Member
Apr 13, 2023
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Hi,

I have a 2TB external SSD drive, and I'm trying to copy its contents to a 4TB external SSD drive.

Both drives were formatted in Exfat using the default Windows settings for allocation unit size (not sure what that means), which are 1024 kilobytes for the 4TB drive and 128 kilobytes for the 2TB drive.

When I compare the space taken by each folder on their respective drive however, I notice that although the folders are identical, the copy I made on the 4TB drive takes significantly more space. It seems that this must be due to the difference in allocation unit size so I'm wondering what settings I should use for the 4TB drive. Or is it a bad idea to format such a large drive in exfat to begin with? (it took me almost 4 hours to copy that 551GB folder using USB2 ports, which seems slow to me, not sure if EXFAT makes for slower transfers or not).

My environment is Windows 11, both drives are Samsung in case that makes any difference.

(as a side note, no matter how many times I change that setting, my folders have always reverted to "read only" for as long as I can remember, even though I can modify the contents without any problem)


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Thanks for the reply. I chose exfat in case I want to access the drive with a mac. If you think it's not a good idea I can reformat.

The 2TB drive (the source) is MBR, the 4TB (destination) is GPT. I don't know how they ended up being different. Which is better?
 
Exfat is needed for cross platform.
For 4TB and larger GPT.
rom1 said:
which are 1024 kilobytes for the 4TB drive and 128 kilobytes for the 2TB drive.
Click to expand...
what happens with fat (file allocation table) formats is always hard to explain.
Take it back to the old 8bit sectors. If a file is 6bits it takes up the 8bit sector.
If the file is 9bits it takes up 16bit sector.
So, you have 2 different sizes 128 and 1024.
So, if a file is larger than the 128 e.g 150 it will use the 1024.
Does this make sense?
If not, I will see if another tech can explain it in a different way.
 
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It's a little confusing but thanks for trying :) I don't understand why file allocation needs to be different depending on the size of the drive. I read a bit about file allocation online and it didn't make a whole lot of sense to me so my computer literacy is probably not that great to begin with.

But rather than trying to understand how it works, the most important to me is knowing whether I used the correct settings and if losing more space on a bigger drive due to different file allocation values is unavoidable.
 
rom1 said:
why file allocation needs to be different depending on the size of the drive
Click to expand...
Because when this all started there was no concept that a drive could be so big.
The first home computer I saw was 2 floppy drives.
When it was upgraded to 80MB (YES 80 Megabytes) hard drive. Everyone said that was crazy.
You could never fill a drive with that much data.
 
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PeterOz said:
Because when this all started there was no concept that a drive could be so big.
The first home computer I saw was 2 floppy drives.
When it was upgraded to 80MB (YES 80 Megabytes) hard drive. Everyone said that was crazy.
You could never fill a drive with that much data.
Click to expand...

My first computer was an Atari ST so I remember those days. But if I'm not mistaken, EXFAT is not that old.

But then what shall I do? Just go with the default settings for file allocation? Is it normal that those settings are that different for a 2TB drive and a 4TB drive?
 
unless the drives are setup the same, than you are going to get different results.
one has an allocation unit size of 1024KBs and the other 128KBs, so whatever files use that space will need a different amount of units depending of the file size and the drive in question.

the end result is, the space used will be different because the allocated unit sizing's are different.

and that's before you even start thinking about fragments, file management overheads, or the files in question. for example; what is the ration of small files vs large files, as the allocated units are used up differently.

see if this explains it better than I can; https://helpdeskgeek.com/help-desk/what-allocation-unit-size-is-best-for-drive-formatting/

unless you compare apples with apples - you will get varying answers.
 
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