Is there a way to speed up the uploads ?
You need to isolate the bottleneck(s).
If it’s a HDD you put into an enclosure yourself (and not an external drive you purchased that comes with the HDD already inside), you’ll get better throughout by installing the hard disk into a desktop computer instead of relying on USB.
If the external drive supports USB 3 and you’re plugged into a USB 2 port on the computer, use a computer with USB 3.
If you’re maximizing the internet connection with the upload, use an internet connection with faster bandwidth.
It’s good you’re backing up, as trusting an external HDD (especially the non-SSD variety, but any really) as the singular storage for your valuable data/images is a risky proposition.