I've got two 4 TB drives, One 5 TB, and a 8 TB drive all using USB 2.0 technology and now this 8 TB running on the USB 3.0 platform all connected to the same system and all runs well. One note, I use them exclusively as Portable Hard Drives, meaning each one shows up in Windows Explorer as it's own drive letter.
I did not like how the software backs up my files automatically and because I have these mostly filled, it was processing the info too slowly and with a PC that I made myself with all the horns and whistles in speed and technology, I noticed too much lag in the over all processing of the PC and it slowed the over all speed down on the PC that it was significantly slow as these back-up software programs typically do.
I just use them as a stand alone drive and send my files to the drives individually when I need them "backed up". Basically, just dragging and dropping said files over to this drive when I want them "backed up" on another drive. You don't have to use the software that comes with the drive and it is not set up yet on the drive, you have to tell it to be an automatic back up drive.
Note: when I started the auto backup program of backing up 6 TB of my internal hard disk, it was running the initial back up for 2 days straight and not finished, so I killed the process and started using it as a drive instead. The software is already installed on the drive for you if you want to use it, otherwise the drive is available to you when you plug it in as another drive letter in your Windows Explorer, it's that simple!