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That's because you're using NTFS in the external disk. Mac OS X can't write on NTFS drives, so you have to erase the disk in order to write on the external disk.
1. Copy all the data from the external disk to the internal drive.
2. Open Disk Utility, select the external disk in the sidebar and go to Erase.
3. On Format, choose Mac OS Extended (Journaled) if you're going to use it with Macs only, or exFAT if you're going to use it with another PC, and press Erase.
4. Copy the data to the external disk