I get an old laptop with Windows 7 installed. Wishing to just clean install Windows 10 while keeping the data on non-system partition, I made a mistake by booting from Windows 10 installer USB drive, removing the system partition right before trying to start the installation. The installer complaint about booting from UEFI and can't install on non-GPT disk. If I haven't remove the system partition, I can easily boot to Windows 7 and use third party tools to convert from there. Since I don't have a working OS right now, Windows PE is my only hope. FDISK can easily convert MBR disk to GPT, but only if the disk is empty, while I need to keep the data on other partition, and don't have enough time nor space to back them up first.
I can download small files from my phone and copy them to the USB drive, but the connection isn't stable enough for downloading a Linux Live USB or paid bootable partition manager. How do I convert the disk losslessly to GPT without downloading large files, and preferably right from Windows PE?