A similar question got asked but got closed.
If you install Debian 7.7 to a USB in Windows with unetbootin and then try to install Debian from that USB, it will complain that it 'cannot mount CDROM' and the install will not go any further.
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I got the same error message when booting from a second partition of my usb stick. Zorbax' solution helps here too, however you need to choose the correct partition. For my case it was
mount -t vfat /dev/sdb2 /cdrom
Acccording to Debian-faq
This might be a little problem on older versions of Windows, as many Windows image-burning programs use their own formats for CD images.
Win32DiskImager is particularly good for pre-release testing and new releases, when the standard tools like Unetbootin might not be ready
Maybe there are an issue with unetbootin on some windows distro . You can use an alternative like : win32diskimager or Refus recommaded by Ubuntu , or ImgBurn ...
In order to install Debian 9.5.0 on the bare metal, I had to enable IOMMU on my motherboard BIOS (for the keyboard and mouse to work). After that, I put Debian DVD 1 ISO into my pendrive with Rufus tool using "dd" mode. To make the graphical installer read the packages from my pendrive as if it were a CD-ROM, I had to plug the pendrive into a USB 2.0 port available at my machine's front panel. And to finish up installation, I said "no" to "force UEFI" option.
(I haven't experienced bare metal install issues with Windows over USB 3, though)