LaCie external hard drive issues with time machine

Hey guys about 2 months ago my hard drive crashed on my iMac and I lost everything.....anyways I swore it wouldn't happen again and I have since installed a LaCie external hard drive that is synced up with time machine. My problem is that every for often time machine stops recognizing the external hard drive and I have to disconnect it and reconnect for time machine to again recognize it. Anyone have any idea as to why this is or how to fix it? Thanks in advance.