Seagate Slim 2TB Not Recognized I recently purchased a Seagate Slim 2TB external HD.  I connected it to my 2010 iMac, formatted it as exFAT, and moved a bunch of files.  I don't remember exactly what process I went through with disconnecting it, but I have never been able to retrieve the files on a mac.
I connected it to my Windows 7 PC at work, and can access everything just fine.
I am currently on a 2014 MacBook Pro running Yosemite 10.10.2

The drive shows up in disk utility, but it wont verify or repair the drive.
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.