used space in a new empty partition

Solution 1:

They are reserved for root and privileged processes. Run sudo tune2fs -m 0 /dev/sdc5 to free them up for regular users.

From the man page:

Reserving some number of filesystem blocks for use by privileged processes is done to avoid filesystem fragmentation, and to allow system daemons, such as syslogd(8), to continue to function correctly after non-privileged processes are prevented from writing to the filesystem.
Normally, the default percentage of reserved blocks is 5%.

If you are just using the partition for extra storage, you don't need the reserved space.