Windows – Why copying files in one partition is much faster than copying files to another partition

I don't think, copying on the same partition is faster. If you mean "moving", then look at the answer of Daniel Schwartz, he explains perfectly what's going on.

However if you mean a real copy-operation, I would not expect big performance differences as long as the operation takes place on the same disk. There may be some differences because of the time, the head-placement-operation will consume for harddrives (obviously not for SSDs), but I dont expect the impact of this to be very big. Maybe there are also some differences because of different caching strategies of the OS, but also this effect, I dont expect to be very big.

What makes a great difference is copying from one to another disc. If the controler is fast, this is much faster than copying on the same disk, because either drive only has to read or to write, not to do both of the operations together (An exception of this rule may be small data, because of the hdd- and controller cache).

Please note that the OS can make links to files. In this case, the data will not be copied, but only a new reference to the data will be created (The file will also not consume double its space). This is very fast. But if you copy your files with explorer or standard shell commands, the files will be copied and not linked.