Poor file transfer Server 2012 Essentials

Hi All, Any help appreciated with this one!!

I have a Windows Server 2012 Essentials Server with Dell Perc 200 raid controller.

All generally works very well. But i have noticed if i copy files from one location to another location on the same hdd, the transfer rate starts reasonable and drops right down to 1-2mb / sec... or less... and sometimes goes to kb/sec and then sometimes drops to 0...

Any ideas or suggestions are welcomed?? Is it windows settings, the raid controller etc???

Thanks, Simon