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If you have a NTFS external drive, you can't write on it without a program like Paragon NTFS. If you don't want to use a program, you have to format the disk.
1. Open Finder and copy all the content of the external disk to your internal drive.
2. Open Disk Utility, select the external drive in the sidebar and go to Erase tab.
3. In Format, choose Mac OS Extended (Journaled) if you're going to use the disk only on OS X, or exFAT if you're going to use it with PCs, and select Erase.
4. Copy the files to the external disk