I have a seagate usb 3.0 external HDD-normally i get speeds of upto 80 Mbps while copying a single 4/6 GB(or bigger)file to it(such as large ISO files)by connecting the drive to my pc's usb 3 port;however when copying several files to it at one go(for instance a folder containing a couple of hundred .jpgs or mp3 files),the speed reduces to around 20-40 Mbps.Therefore,it can be concluded that the write speeds may vary depending upon the type of files you are trying to copy.
Once i had used it to copy some data from my friend's pc,which has only usb 2 ports-there the copying speeds never went beyond 10-25 Mbps.So I think your pc's USB 3.0 ports are functioning properly,otherwise you could never have achieved such high speeds while copying files(i.e. 40-50 Mbps);you have mentioned that you obtained such speeds even while using USB 2.0 but i don't think its possible-I have used several usb 2 drives till date but i never got 40-50 MBps on any occasion.