Copying a file returns "No space left on device"

Issue

• Copying a file returns "No space left on device" even though there is space:

# df -h
Filesystem            Size  Used Avail Use% Mounted on
/dev/sda3              56G   26G   27G  50% /
/dev/sda1              99M   25M   70M  26% /boot
/dev/sdx1              50G   40G  7.3G  85% /mountpoint

$ cp /path/to/sourcefile.tar.gz /mountpoint/directory/
cp: overwrite `/mountpoint/directory/file.tar.gz'? y
cp: writing `/mountpoint/directory/file.tar.gz': No space left on device