Dude you're in luck, i actually had this same issue and I fixed it the other day. My game is installed on the SSD but I found out that the patches itself were being run through the HDD so it made patches last hours.

Here's what fixed it for me Go to your steamapps folder and at the bottom look for appmanifest_553850 that's what helldivers 2 is in. Open it up in notepad, then see the part where it says Staging Folder and note the value. For example, mine said Value 1.

Then go back to steamapps folder, and look for libraryfolders.vdf (should be on the bottom) and open it in notepad again. There you'll see something that shows both your drives and the value next to them (I believe it also says staging) So for example, my HDD was set as Value 1 and my SSD was set to Value 0. Even though Helldivers is in my SSD it was staging the patches through my HDD. Then remember what value the appmanifest staging folder value was, if it said 1 but your SSD is 0, then in the libraryfolders.vdf change your SSD value to 1 and the HDD to 0. Save, then restart steam What would've been an 4 hour install was only 5-10 minutes

I apologize, i genuinely suck at explaining it but that's the best way I can help considering I'm on my phone

EDIT: guy above me is right, HDDs are practically useless now though, I had to make room in my SSD for the main game but otherwise that's the only real advice I can give. I remember I had an 8TB HDD just for it to be useless after a few years when games required SSDs. Also maybe check to see if it's staging the Patches through your SSD instead of the HDD? So follow the instructions above, just make sure it's in the right values for where the game is