I have 2 HDD's, C: drive, which is used for the installed Windows 7 and other not greatly important stuff and D: drive which is used for games, movies and photos. I'm considering buying a SSD and having a clean install of Windows on it. Can i unplug my D: drive, get a fresh install and replug it to keep my files if i install Win 7? If i install Win 10 will it cause problems?
Well no you don't need to delete everything "manually" if you mean go through directory by directory and file by file. Once you have a functional SSD with an OS on it take your original C drive (which you do not connect till AFTER boot so as not to get the PC confused, your new SSD is going to be the C: drive with the OS).
Win X > Disk Management > right click on the old drive select "format", then NTFS, Default, "perform a quick format" all of which will likely be preselected. If it asks make a boot drive (I think it will see the disk used to be a boot drive) say no.
That will clean things up and incidentally make it so your old drive is no longer seen as a boot drive. For the purposes of ordinary people all the data will be eliminated. The FBI could pull some data off such a drive, but for us, it's done. Then you can use it to store your files.