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Modern Linux distributions typically have LVM resting between physical storage devices and the file system.  This is great for your OS and other data drives that are installed in your system or otherwise "permanently" attached.  The ext file system is designed to allow resizing as the underlying block devices are grow (or even shrunk in the right scenario).  But this does require that the file system reserve space to allow what's called the "block group descriptor table" to grow.  USB drives don't magically change in size (outsize of you messing with partitioning) and because of this don't need to reserve that space.

To get the most file system bang for your buck out of your USB drive use the following mke2fs command:

# mke2fs -m 1 -O ^resize_inode -j -t ext4 /dev/sdz1

Replace /dev/sdz1 with the path to the partition on your USB device.

The -m 1 argument reduces the amount of space reserved for the super user to 1%.

The -o ^resize_inode argument disables the reserving of space for resizing the file system.

Check out the defaults in /etc/mke2fs.conf and man mke2fs for more information about how your file system is being sped up.