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.
You can mount manually in a shell window (Ctrl + Fnumber)
mount -t vfat /dev/sdb /cdrom
But, the option of warsong works fine. It seems like a UNetBootin-Win bug u_u
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)
The way around this problem is to rename the Debian .iso file to a .img file, and then to write the image to the USB with Win32DiskImager (I used 0.9.5 and it worked fine). The install will then work.
For QEMU (if a switch like "-cdrom installer.iso" is used):
mount /dev/vdb2 /cdrom