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My file system claims to only have 4.5 GB left. While my OS (a folder with in file system) still has 75.2 GB left. I put something near 130 GB on my Ubuntu partition, it should have enough space. I confirmed that I can put things in OS that exceed the space in available file systems, but that makes no sense, OS is listed as a folder inside of file system, why would it have more space than it's parent folder? What is going on?

Here is the output of df:

Filesystem 1K-blocks Used Available Use% Mounted on 
/dev/sda5 113773200 103741440 4252408 97% / 
udev 2004600 4 2004596 1% /dev 
tmpfs 804756 848 803908 1% /run 
none 5120 0 5120 0% /run/lock 
none 2011884 436 2011448 1% /run/shm 
/dev/sda2 127526908 54045584 73481324 43% /media/OS 
/dev/sda3 39144708 89016 39055692 1% /media/DATA`
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Add the output of df to your question. – psusi Dec 2 '12 at 1:25
The way you keep referring to file systems without identifying them precisely and mixing folders with mount points makes your question an excercise of finding out what you actually meant. Root fs has ~4GB of free space and resides on /dev/sda5. From the information you provided, 'OS' is not a mere folder within an FS but a mount point for the /dev/sda2 partition which has over 70GB of free space. What are you referring to as "your Ubuntu partition"? It looks like you're confusing folders, mount points and partitions. – Marcin Kaminski Dec 2 '12 at 3:10

/media/OS is a separate partition /dev/sda2 and does indeed have 73.5 GiB left as free space.

/ (root) is on /dev/sda5 and is 97% full - 4.25 Gib available. It looks like your /home is part of the filesystem on /, so data files that you place in there will take up space on /

This is why I usually have a small but sufficient paritition for / of say about 25 GiB, and make the rest of the drive available an mounted as /home. I can fill home up as much as I like, and it won't affect the operating system on the root file system.

You will be able to free up some space on / by moving some of the data files you have in /home onto the partition you have mounted as /media/OS. I've never been a fan of multiple data partitions for no clear reason since unexpected low usage in one and unexpected high usage in another leaves you in a rather silly situation.

In short, it's working this way because that's the way it has been set up. Another solution might be to resize and move some partitions around to better fit in with the way you are trying to use them.

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