Even though i read transfer speeds of 360 MB/s, peaking at 900 MB/s, it now took more than six hours to copy a part of one Terabyte, and it's still running, being half way through. I know, that copying lots of small files, in my case images, often takes longer, than copying large files. But according to my calculation, the transfer should have been finished after less than one hour, if i assume, that one GByte takes around three seconds. So the real average transfer speed has to be much lower.
Transfer speeds with lots of tiny files will be largely meaningless. Most of the time will be taken creating the files (which is very slow on Windows), not copying the actual contents of the files.
Thanks for explaning the reason. I already suspected, that Windows' copying speeds are still the issue, regardless, if it's USB 3.0 or not. At least it's fast with really large files.
I now have found the reason for the slow copy speed. My new external harddisk was formatted with ExFAT, which seems to be some kind of new fashion. I didn't pay attention at first. Now, having it converted it to NTFS, the copy speed is significantly higher. Maybe at least twice as fast, even though the copy speed info stayed the same.
After i have finished copying my images, i am now transferring my music collection, and now the speed is very high, for example, i was able to copy a 20 GByte in less than 5 minutes, whereas the same amount of images would have taken about 30 minutes. So the reason must have been all the overhead in the data, with lots of tags and metadata like coordinates.