Krdytkyu

I made a bootable Windows 7 USB using UNetbootin, and when I try to boot from the bootable USB and select the Default boot menu entry, it gets stuck at the UNetbootin boot menu and it keeps repeating an Automatic boot in 10 seconds... message in an endless countdown loop. Selecting Press [Tab] to edit options and editing the options has no effect. When I go back to the boot menu, it keeps repeating the same countdown loop.

I've tried redoing the UNetbootin extraction, but I still have the same issue.

The "Automatic boot in 10 seconds..." countdown loop is a UNetbootin problem. If the UNetbootin boot menu does not have an entry for the OS you are trying to boot to under the Default boot menu entry, this is another sign that UNetbootin is not working properly. I know that it's a problem with UNetbootin because I have seen it before, and managed to solve it by reformatting the USB flash drive to remove to bootloader and then making the live USB again with Startup Disk Creator. Startup Disk Creator is capable of making both Ubuntu and non-*buntu bootable live USBs.

This wouldn't solve your problem because you are trying to boot Windows 7, so I suggest that you use the WinUSB application to make the bootable Windows 7 USB flash drive.

First reformat the USB drive as FAT32 to remove whatever obsolete boot information UNetbootin wrote on the USB flash drive. If your computer detects obsolete boot information on the USB flash drive, it may try to boot an operating system that no longer exists on the USB flash drive, and then UNetbootin will get stuck in a repeating countdown.

Then download WinUSB for Windows and install it. Native UEFI booting is supported for Windows 7 and later images (limited to the FAT filesystem as the target device). WoeUSB is an updated fork of the WinUSB project.

Some third-party installers feature Windows installation images (/sources/install.wim) greater than 4GB making FAT32 as target filesystem impossible. NTFS filesystem support has been added to WoeUSB 3.0.0 and later.

To install WoeUSB (updated fork of WinUSB project) in Ubuntu 14.04/16.04/17.10/18.04 and later:

sudo add-apt-repository ppa:nilarimogard/webupd8 

sudo apt update  

sudo apt install woeusb

To install WinUSB in Ubuntu 14.04-17.04, open the terminal and type:

sudo add-apt-repository ppa:nilarimogard/webupd8 

sudo apt update  

sudo apt install winusb  

This will install the WinUSB graphical interface and the WinUSB command line tool. The WinUSB GUI is much easier to use than the WinUSB command line tool.

Installing WinUSB on EFI-loaded Ubuntu will uninstall the grub-efi packages in order to install the grub-pc packages, so before you reboot run the following commands to repair grub:

sudo update-grub

sudo grub-install /dev/sdX # replace X with the letter of the partition where grub is located

sudo update-grub

sudo reboot

Just reinstall UNetBootIn and it should work fine. I just did that myself. I realize that your question has probably been solved, just putting this here for the other people that may come across this thread.

I got stuck with the same error after all windows installations completed(my previous os has gone already) and I used these below steps.
1. Remove the USB first,
2. Restart the system. Now go to the boot settings and enable the safe boot mode on.
I hope it'll work for you too.