Problems with Mac and Seagate Goflex Freeagent

I have a Macbook pro and a Seagate Goflex freeagent external hard drive. The same hard drive is used on a pc as well. The problem is I ejected the ExHD from the mac book and plugged it in to the PC. I was able to use the ExHD, I save the info and went an safely removed hardware. When i went back to the Mac I get a message that says the device was ejected improperly and the Mac Osx will fix the problem, however It does not fix it. I can get it to reconigize the ExHD and I eject it and restart the Mac. I can access folders on the ExHD, but when I go to open a file I get the same error message. I had this problem once before. I got the Mac to eject the ExHD and I restarted and all was good. Any help is great. Thanks