Seagate Slim 2TB Not Recognized
Plug it into the PC, make sure it mounts and you can read and write to the drive, make a note of the drive letter.

Open a command prompt as administrator and issue this command

chkdsk x: /F /X


the "x:" is the drive letter of your drive. /F tells it to fix any problems and /X tells it to dismount the disk so it can work on the drive.

When that is done try it on the Mac. If it still fails to mount, plug it back into the PC, copy all of the files off of it and delete the old partition and add a new one and format it as desired.

Copy the backed up files back to the drive and plug it into the Mac. If it still fails to mount, return the drive for another.