I’m trying to install Debian and boot from my USB but I always run into a problem that says “Failed to copy file from CD-ROM. Retry?” and “The CD-ROM you have inserted is not a valid Debian CD-ROM. Please change the disk.”
https://cdn.discordapp.com/attachments/371830397006381080/388869309591453719/IMG_20171208_151626.jpghttps://media.discordapp.net/attachments/371830397006381080/388869262573568000/IMG_20171208_194959.jpg
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How good is your Internet connection? If you’ve got at least 10 u/d you should do the netiso.
https://www.debian.org/CD/netinst/
Using the CD-ROM or DVD installer has always failed for me. The netiso is one image and it downloads the components needed to complete the install.
Are you making the disk on Linux or on Windows? If on Linux, try using the Gnome-Disk-Utility.
Hamburger Menu -> Restore Image -> Select Iso -> Restore
If on Windows, try Rufus. Be sure to specify GPT or MBR on Rufus before you create the image.
Hope this helps 
Debian Stretch is a great distro, only ran into one issue my entire time using it.
Some programs for making an install usb have that problem lili and unetbootin come to mind. Might have to try a different program or get a usb.img.
I don’t recomend what I put under this as it is mostly just there for entertainment value and is something I had to do once when I had no operating system installed, basically I formatted my Hard drives and then I ran into that problem.
Another option that sometimes works but I would really recomend -->not doing<-- is pulling the usb and plugging it back into a different usb slot when it asks for the cd, later you will get an error about not being able to locate the install CD and if you plug it back into the first usb it might work. By might work I mean somewhere around 1 in 4 tries so better to just burn another img or iso to usb. Mostly just put this here because it was something funny I had to do once.
