I just built a PC and accidentally installed Windows 10 on the HDD instead of SSD (my SSD wasn't powered when I was installing the OS). I then disconnected the HDD and installed Windows 10 on the SSD. When connecting both SSD and HDD, only HDD displayed on 'My PC', and it wouldn't allow me to delete Windows 10. How do I wipe everything on my HDD since I already have Windows 10 on my SSD?
You can simply reformat the HDD if there's nothing on it that you want or need.
I just gave this same piece of advice for another issue, but check your boot order in UEFI/BIOS as well. Be certain that the SSD is listed as the first boot device. My guess is that the HDD is still considered the primary boot device and, when they're both connected, you're actually not booting from the SSD as you think you are.
It's now blessedly more difficult to do something bone-headed, like in the early days, when you could just run a format on your main system drive while it was your main system drive. That had some "interesting" results.
TBH, likely the easiest method would be to unhook the SSD. Put in your install USB and go in like you are going to load Windows. Use the advanced option to see the disk and delete all the partitions there, stop the install, hook the SSD back up and you should be good. You will have to go into disk manager to allocate the space on the HDD.
Start from scratch. Disconnect the HDD and install the SSD.
With only the SSD connected, format the SSD and install Windows.
Once the install is complete, shut down and reconnect the HDD.
Restart your system (should start from the SSD) and you should now be able to format the HDD.