If I open it in Linux, I get:
If I open it in Windows, I get:
"setup"tries to install Windows.

I tried copying the Editions exe from the DVD to a thumbdrive, but Linux said that it could not read the file

Flash drives are typically automatically formatted as exFAT or NTFS. I don't think debian automatically supports either one so you need to install ntfs-3g or exFAT fuse.

Does your Windows guest have a virtual optical disk drive? I am only guessing that your not connecting the USB drive to your guest and that your looking at the wrong drive from Windows. The virtual drive contains the Windows installer? You should be able to "remove" the Windows installer ISO file and "insert" the guest ISO file.

I mentioned how to connect a USB device a few posts back. From the guest's window pull down menu select
Devices -> USB. There should be a list of recognized devices and select the USB DVD drive. There should be a green check mark on the left. A device should automatically be created and from the file browser select the optical drive letter. You should now be able to see the real DVD contents.

I have an optical drive that attaches to the computer via USB. I downloaded the VBAdditions.iso using Linux; burned that onto a DVD (51 Mb won't fit onto a CD), and mounted/opened the drive using either Microsoft or Linux. Yes, Microsoft does recognize that drive.

Expanded contents of the ISO:
The directory labeled "boot" contains three directories: en-us, fonts, and resources.
the directory en-us contains one file: bootsect.exe.mui
the directory fonts contains 18 *.boot files and two *.console files
the directory resources contains two .dll files

The directory labeled "efi" contains two directories: boot and microsoft
The directory labeled boot contains one file: bootx64.efi
the directory labeled microsoft contains one directory: "boot"
that directory boot contains three directories: cipolicies, fonts and resources, and also 7 files
those seven files are: bcd, cdboot.efi, cdboot.noprompt.efi, efisys.bin, eficys.noprompt.bin, membest.efi and winpolicy

the directory /sources/dimanifests contains a whole bunch of stuff, mostly .dll files.


Bottom line: I don't see anything resembling a *.exe file.

I probably am not explaining correctly however you are not providing much feedback so I must infer a lot. It appears like you burned the ISO correctly but the difficulty is in trying to get Windows to "see" the USB drive and optical disk.

It appears like the Windows installer ISO is still inserted in the virtual optical disk drive. The contents you post look like the installer disk. To remove the ISO installer select from the guest pull down menu Devices -> Optical Drives -> Remove disk from virtual drive

Once the Windows Installer ISO is removed you should be able to "insert" the guest additions ISO. Devices -> Choose a disk file. Select and open the guest additions ISO file.

You need to manually connect the USB drive to the guest using the steps posted above. If Windows recognizes the USB optical drive with the disk inserted then you should have two drive letters corresponding to each optical drive. If you only have one virtual hard disk then the virtual optical drive would be d: and the USB drive E: By switching between the drives you should see different contents. If possible post a screen shot including the file browser so we might see exactly what is happening. If you do not have separate optical drive letters the USB drive is not actually connected to the guest.

GOT IT
I could not get my head around that it was reading from a disk that was not plugged in.
THANK YOU!