Discovering that I was low on space on a partition I'm using for recordings, I went to gparted to move some things around to get some more space until a new hard drive arrives. When I did this, I discovered that gparted shows that there was almost 7x as much free space as reported by the file manager or the df command.

gparted shows:
Partition - Mount Point - Size - Used - Unused
/dev/sdc9 - /mnt/Recordings - 260.80 GiB - 245.62 GiB - 15.18 GiB

df - h shows:
Filesystem Size Used Avail Use% Mounted on
/dev/sdc9 257G 242G 2.2G 100% /mnt/Recordings

It's not a root or boot partition, and I don't see this large discrepancy on my other partitions, so I'm curious about it.

The only suspicion I have is that, since I am sharing this directory to access it from multiple devices, there is some Samba/smb overhead due to that sharing that I'm missing and don't know how to identify what that might be. Can anyone explain what I'm looking at here?

Thanks in advance.

if it was accessed from several devices it could explain this behavior. You need to detach (disconnect/unmount) if you want to be sure. Also you may need to take the cache into account.

If this is an ext filesystem, I suppose df takes the 5% of reserved space into account.

How much space do you need?

As posted it looks like the drive formatted as ext3/4 which means that 5% is reserved for root which matches information provided. Reserved space is not included in the available space output of the df command but does in the unused space output of gparted.

Reserved space is supposed to reduce fragmentation as well as allow root to log in case the filesystem becomes full. With a data partition the latter does not apply. You can reduce reserved space to zero if necessary which is about 13GB.