My laptop came with a 256GB SSD, which houses Windows 10, and a 2TB HDD. I put Manjaro Linux on the HDD back when I first got the laptop.
After an update yesterday, Windows keeps giving me the BSOD, and it's looking like the easiest way to deal with that will be to reset Windows. I have backups of everything I need.
My question is, will resetting Windows affect what's on the HDD? I.e., will it end up removing Manjaro, or will it only affect the SSD?
No, it should not do anything to the other drive. It will clear your main hard drive, the SSD and reset it back to factory defaults. But just in case, disconnect your HDD so whatever this "reset" does, can only physically affect what is connected. In this case the SSD.