Well both USBs and hdd rpm speeds play a major part in hard disk drive speeds. A solid state drive (ssd) connected to a usb 3.0 port can have a write speed of up to 250mbps while a 5400rpm hdd while still connected to a usb 3.0 may have a write speed of 45mbps so the external hard disk drive speed may be higher depending on the HDD rpm speeds. Usb ports also have a part to play in write/read speeds. If a 5400rpm hdd averages 45mbps on a usb 3.0 port, its write speed would also reduce to about 22mbps. I hope this is helpful bro. Cheers..