Storing files in the partition containing OS

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http://askubuntu.com – My hdd is partitioned into 220gigs n 250 gigs approx.Im able to store files on one of these partitions but I cant store any files on the other partition having the OS.The paste option never appears.I would like to use this space to store some files temporarily.Could you guys help me?If this question was asked previously Im extremely sorry. Thanks in advance folks (HowTos)