Mac won't eject external hard drive

Hi!

I've a MBP 15-inch Early 2013 256 GB HD and 8 GB Ram, with Yosemite 10.10.2. A year ago I'd problems with the logic board and Apple changed it, two times. Now I've problems when using my external hard drive ADATA Classic CH11. When I connect it everything works fine, Time Machine does the backup, I can transfer files, no problem. When I try to eject the disk it grays out from Finder but stays like that and never ejects. When I try to open disk utility it freezes and I need to Force quit it. If I try to shut down the Mac after that it never does, it closes all programs but stays on without shutting down.

I've searched about it but all the people say that there's something wrong with my Logic board, this is the third one I've so I'm not sure I'll be able to convince Apple to make another exception to change it.

Any ideas?

Thank you!