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.