In this howto there are the steps to follow when a Windows 10 OS is not bootable anymore and you haven’t a recovery disk. This is a typical case after a new OS will be installed on Dual boot or boot partition was altered.

1. Download Windows 10 iso:
   1. Download the official Windows 10 image
2. Prepare USB to be bootable:
   1. Open GParted with

gparted /dev/DEVICE-TO-ERASE

Select the USB drive

Device > New partition table

Select GPT

Apply: this will delete any data on the USB

Create a new NTFS partition then Apply (do not use FAT32 since some files can be greater than 4GB)

Close GParted

Write files:

1. Unplug and plug USB
2. Copy all Windows files to the empty USB drive using 7zip with:
   

   7z x -y -o/media/user/path-to-USB/ Win10_1809Oct_Italian_x64.iso

3. If something goes wrong during copy, you can mount the ISO image then rsync the source with the USB drive (the trailing slash is important):
   

cd path/to/usb/drive
rsync -avzp /media/myuser/CCCOMA_X64FRE_IT-IT_DV91/ .

umount

Add boot flag

1. Open GParted selecting the device just written
2. Select the new partition then
3. Select Partition > Manage flags
4. Select boot flag (esp will be auto-selected)

1. Use windows tools
   1. Follow this howto by MS to recover MBR, restore BCD or similar actions

You can follow these steps to write on a USB a recovery ISO from windows the same way.