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ext4 partition size / free space discrepancies

The results from a df on my main drive on my server don't add up. I came close to running out of disk space, I freed 4 gigs a couple of days ago, but we're still getting weird results two days later.

Filesystem           1K-blocks      Used Available Use% Mounted on

/dev/sda1             78441416  73732352    724536 100% /

Now, by my calculation, 78,441,416 - 73,732,352 = 4,709,064, so there should be over 4 gigs left, not the 724,536K that df is reporting.

I don't see this could be a quota error - though quota -f apparently isn't an option for Ubuntu and I haven't yet found a command to recompute the quota.

I'm now going through the various directories doing a du -sk (glad the server seems to have light traffic for a Monday morning) but not sure what that will tell me.

Any hints?