Last month I bought new external SSD drive of 512 GB. I plan to use it as a file history backup of my Windows 10. Initially, I have no idea why I left with only 25 GB free space after I activate the file history. Until this morning when I really need more space, I started to find out more what is actually happening.
I right click a folder in C:\ and the compare it with the external SSD, and found out that the ratio between Size and Size on disk are different between both. In C:\ , the Size on disk is typically 10% higher than value of Size – however in my external SSD the Size on disk is 300% higher of value of Size.
Running following command,
wmic volume get driverletter,blocksize
As we can see, that the external SSD is formatted with 132 KB blocksize as opposed to 4KB blocksize in other drives. I also notice that external drive is using exFAT not NTFS. I read about exFAT vs NTFS here – https://www.howtogeek.com/235596/whats-the-difference-between-fat32-exfat-and-ntfs/ , and decided that I will use exFAT so that the external drive will still be portable.
Formatting exFAT file system with 4KB allocation unit size
Although technically I should be able to format exFAT with 4KB allocation unit size, unfortunately Windows 10 does not provide the GUI. In the GUI, it only provides option to specify at least 32KB allocation unit size for exFAT.
So, I open my command prompt and start using format in command line.
format G: /FS:exFAT /Q /A:4096
Now when I compare Size on disk vs Size , the different is about 10% . So, next time you feel that you lost too much free space too fast, may be you should if you choose appropriate allocation unit size.