I'll try and help you.
There is a difference between mbps and MB/sec. Megabytes per second (MB/sec) is what hard drives copy at.
I am in the process of copying some stuff to my Mom's Laptop (crappy 5400rpm laptop 2.5" drive is rather slow by most standards) above. You can see I get MB/sec. Not mbps (usually this measures internet speeds or network speeds but it's not the same as MB/sec)
It does not matter which we use, but we should be clear about it. You can convert them back and forth using google browser which does a nice job with such things.
Most Hard drives are MB/sec- with slow drives being about 60MB/sec and fast drives being about 160MB/sec. All sorts in between. Below 50MB/sec is pretty slow for a modern drive. The item you copy matters too. For example if I copy MP3 or a folder with installation files it will copy much slower than if I copy a 25GB movie. Big single files always move the fastest- Sequential reads and reads are faster than random smaller reads and writes on all drives.
You can easily test your hard drives and your LAN speed with different benchmark programs. I would start there. That will show you exactly what speeds you get from each hard drive, and also your LAN speed too. From those results we can see what is wrong with what and work about fixing it.
Start by downloading a hard drive benchmark utility (google it) and measure your read and write speed. Post the results. Also do the same for your LAN connection too. We can figure this out.
WD GREEN 5400rpm drives go about 100MB/sec down to about 60MB/sec from my experience. They might get a little slower than that if totally full, but usually that is the range you get. Seagate 4TB or 3TB are usually faster, but if you have it formatted wrong, or there is something wrong with the drive that might mess things up.
As a rule of thumb 5400rpm are usually 50MB-110MB in speeds and 7200rpm are usually 80MB-160MB in speed. But results can be all over the place depending on different circumstances. You only need 25MB/sec to stream HD movies though....