Stats below are with a i7-3960x
Robocopy was introduced by Microsoft with Win 7, it does NOT work with earlier versions. It is an EXTREMELY fast copying utility. A test I did just before writing this so I could quote "live" stuff, copied 3.1Gb in 17 seconds .....
On that copy session, DOS reported the transfer as 179,435,063 Bytes per sec .... or 10267.356 Mega Bytes/min.
So, to Robocopy a Disc of 100Gb would take me around 9mins 20secs (not days ....)
The above example times are with an i7-3960x running the command inside a DOS box, so scale according to processor you have. Its still blindingly fast, whatever way you cut it, whatever processor you have, compared to other copy commands.
The Robocopy command will use Multi-Threading, hence the speed. Some of the command options need a little Focus by the user as the syntax is different from classic DOS commands - but the result is well worth the investment of time.
The full Robocopy command set can be seen at:
http://en.wikipedia.org/wiki/Robocopy
Run, don't walk to get up to speed on Robocopy if you are not aware of it (and have Win V7 as a minimum) .....
Make sure you read the guide properly ....... don't assume the Syntax is like all other MS copy-like commands, it's not - READ THE WIKI ARTICLE QUOTED ABOVE