I understand that but don't you think that 18-22mBps is little too low? I got over 100 mBps read as well as write speed in crystal disk mark so technically shouldn't I get about 50mBps even when I am...

So do you mean that it is normal?

Thanks for your reply.
I know the bits and bytes stuff. I really meant 70-80 megabytes per second. I have only talked in bytes everywhere.
Do you think 70-80 mBps in a 5900 rpm hard disk are not...

its a Seagate 2TB 5900 rpm drive. Model no. is ST2000VM002-9UY166 2000.3 GB.
Whenever I start transferring, I get about 70-80 mBps for the first few seconds but then it drops down to a constant...