I've got a notebook with a 2Tb HDD. I'm going to replace it with an internal SSD, and buy a case for the HDD. Then I can plug the old drive into the machine for the extra storage.
However, probably the greatest performance upgrade is free. It is junking Windows, and installing Ubuntu Studio.
Yeah that's doable since you can insert the HDD in the CDs reader caddy , yet you should be aware that the reader you'll be plugging your HDD into is certainly much slower than the one it's currently on so you'll be going from a SATA III to a SATA II port or something similar and thus will have an even slower HDD in the future ..
Browsing the net showed that the best thing to do is to buy an NVME M.2 ssd since it'll be much better than the regular 2.5 inch ssd and keep your hdd in it's place .
Edit : and let's just not go into the windows vs ubuntu discussion , because using windows is really against my will since i'll have problems when working on projects with teams that solely use windows and refuse to switch to ubuntu ..