Its probably formatted as ntfs so you have 2 options
The Mac can read ntfs drives but not write to them.
You could get your stuff off and format is for mac OSX (only readable on Macs or reformat it as exFat (readable an Mac and PCs)
The other alternative is to keep it as an NTFS drive and install components that allow OSX to read and write, 2 options are MacFuse from Google or Paragon NTFS
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