No, that won't work. You will need to boot the windows installer from Grub as explained in the link I provided. You can boot the Ubuntu USB and put a boot entry for the windows installer but when you reboot, it will not be there as the 'live' USB is read only and all data/changes are lost on reboot. You need to put the windows installer boot entry in the grub.cfg file on your installed Ubuntu which means that drive will need to be attached when you boot the windows installer
There are 2 major steps involved, one is creating the partition for the windows installer and copying all the folders/files from the extracted iso to the ntfs partition so that you can boot it.
The second step is the actual installation.
If your Ubuntu install is UEFI, you need to install windows UEFI or it won't boot from Grub. This means your drive, the one you want to install to (500GB drive) should be a GPT drive. You can do this with Gparted if you have it installed on Ubuntu. Otherwise you can use the Ubuntu install USB which does have GParted. You should have an EFI partiton on this drive. I don't know if you need to create this in advance or if the windows installer will do it for you. When I did this 6 years ago, it was on a Legacy/msdos drive.
If you have free space on your Ubuntu drive, I would create an ntfs partition on that drive for the windows installer. If you don't have free space, you will need to shrink the Ubuntu partition and you will need to use GParted on the 'live' Ubuntu USB as partitions cannot be modified from a running Linux OS. Extract the iso and copy the extracted files to this partition and put the windows boot entry in the grub.cfg file so you can boot the windows installer partition. Reboot the machine and select the windows installer entry to begin the installation. I expect the installer will create the necessary partition but am not sure. If you get an error/errors, you may need to create and EFI and ntfs partition from GParted. Once you have finished the installation and successfully booted windows, you can delete or format this partition to use again.
It should not be difficult to get the correct device to install to as your Ubuntu drive is 1TB and the drive you want to install to is 500GB. Use sudo fdisk -l or sudo parted -l to get the naming for each drive (sda, sdb,sdc, etc).
If you successfully boot the windows installer, one of the steps gives you install options. If I remember correctly, this is immediately after the window where you see the windows licensing agreement and here you should select the Custom option if you see it.
I found the link to the site I used to do this. The site explains how to create a bootable windows USB without using any specific software for this purpose and booting it from Grub. THe link is below and remember that it is explaining how to boot from a USB so wherever it makes such a reference, change it to the installer partition on your drive. Also, obviously skip the part at the link below to install Grub as that is not necessary, you already have Grub on the 1TB drive and are not using a USB. Another thin, I would use the UUID method rather than the drive label. This is explained about half way down the page.
https://www.onetransistor.eu/2014/09...om-ubuntu.html
If something is not clear to you, don't proceed but post back before continuing. Good luck.
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