USB 2.0 and 3.0 speed comparison for HDD

So I have a Synology device, with both USB 3.0 and USB 2.0 ports. I wanted to add on additional HDD devices for backing up data. Now my problem is that the USB 3.0 has already been utilized now I am left with only USB 2.0.

My question is how much speed difference would it make? As far as I know the USB 3.0(4800mbps) is ten times faster than USB 2.0(480mbps), but again, the hard disk is a standard one with a write speed of only around 100mbps. So would it make any speed difference if I plug in either ports?

You’re confusing Megabit and Megabyte. USB 3.0 does 5000 Megabit/s, USB 2.0 does 480 Megabit/s. Regular hard drives (spinning rust) can do 100+ Megabyte/s. That translates to 838+ Megabit/s.

Additionally, USB 2.0 tops out at ~40 Megabyte/s user data because of protocol overhead and whatnot.

Using your hard drive with a USB 2.0 port would severely limit its performance. As Gantendo recommends, you should use a USB 3.0 hub instead.