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
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.