Sounds like the optical drive might be defective... You should attempt clean installation via USB. At this stage I would recommend arming SecureBoot and UEFI. Then clean installing Windows 10 RS1: [URL]  [URL]  
Hello, So; I have a Dell Inspiron 7720 laptop with a reformatted hard drive and I need to reinstall windows 7. I do not have the original Reinstallation DVD that came with this exact laptop but I do have one from another Dell Laptop ( a Studio 1558) and another Windows 7 disc from a desktop (the discs and the OS that was on the laptop before the HDD fail are all Windows 7 Home Premium 64Bit). When I turn on the laptop with either disc and mash F12 and then select "CD/DVD/CD-RW Drive" from the boot options the screen immediately displays "Operation System not found" and stays at this screen until restarted. I have run a full diagnostic check and everything passed, I have made sure the CD-RW Drive is no.1 in boot priority (don't imagine that matters if I  manually select it). The HDD and CD-RW Drive are detectable in BIOS too.... I have no idea what the problem is, any help will be greatly appreciated!! Many thanks in advance, 
Attempting to reinstall Windows 7 on an Inspiron 7720- Error "Operation System 
not found"