1st there is an edit button 😀
2nd, I still doubt it's a legit copy or it wouldn't ask for another disk. We can't help you with this. If you had a legit copy you would just be able to insert that disk (or if you made an image, switch images).
3rd if you download games from sites you are probably breaking copyright laws and thus are a thief but your morals could easily allow it 😀
4th, I do have a 3.5" usb drive to make images for those disks, I did this also with most 5 ¼ disks back when I had a machine with such a drive. Old games I buy these days that come with such disks are getting looked at mostly. If it is on GOG.com I'll just rebuy the game, gog is so cheap that I don't mind.
5th legitimate websites offering these games are few. Gog.com, EA's Origin, Steam are good places, some sites have official freeware. But in most cases, when the site mentions abandonware, or has a lot of free downloads, you can be sure it's not legitimate...