Check the file system the drive is using under Disk Utilities.
It has probably been formatted as NTFS since you're able to read the files but not write them.
You'll need to Erase the drive under disk utilities and give it the MS-Dos format in order to write files to the drive. If you're ONLY going to use it on a mac then choose Journaled.
Let me know if you have any other questions.
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