Seagate expansion hard drive setup for mac

Do you have any data on this drive? If not the easiest way to fix this is to format the drive.
Remember that if you share this drive with Windows computers it should be formatted in the fat file system in order to be read by both...
Applications >>> Utilities >>> Disk Utility (select the drive) >>> Erase >>> select file system type (in the format menu) >>> erase

NTFS will read in both Windows and OSX, but won't be writable in OSX (which is probably what happens here). There is solutions available to remove the write protect on NTFS volume in OSX. Check for Paragon, Tuxera or MacFuse.

Format as FAT will read and write in Windows and OSX, but only files individually smaller than 4GB.