If your Windows installation is corrupted and you don’t have a bootable USB or DVD to perform a clean install of Windows 8.1, you can use your Mac machine to create bootable Windows 8.1 USB drive without having to install third-party utilities. One can use the native Boot Camp Assistant software that ships with all recent versions of Mac OS X to prepare bootable Windows 8.1 USB and then perform a fresh install of Windows on your Mac or PC. This method is helpful especially if your PC is unbootable and you don’t have a DVD but ISO file on your PC or external hard drive. If the ISO file is on your PC’s hard drive, you can use tools such as Redo Backup and Recovery, or you have the Windows 7/8 bootable media, you can use the easier how to backup data without booting into Windows. NOTE: You must have your Windows 8.1 product key. If you have purchased Windows 8.1 from Microsoft Store, search your email inbox to find a mail from Microsoft Store containing Windows 8.1 product key. And if you can’t remember or find your Windows 8.1 key, follow our how to easily recover Windows 7/8/8.1 product key from an unbootable PC guide for step-by-step instructions. Step 1: Turn on your Mac, open Launchpad, click Other, and then click Boot Camp Assistant to launch the same. Boot Camp Assistant can also be launched by typing Boot Camp in spotlight search and then pressing Enter key. Step 2: Once the Boot Camp Assistant software is launched, click Continue button. Step 3: Here, check the option titled Create a Windows 7 or later version install disk. If the option Remove Windows 7 or later version is already checked, please uncheck it. And if you’re preparing this Windows 8.1 bootable USB to install Windows 8.1 on a Mac, then you need to check the option titled Download the latest Windows support software from Apple option as it includes all required drivers. Click Continue button to proceed to the next step. Step 4: Click on the Choose button to browse to your Windows 8.1 ISO image file. Select the ISO file and then click Open button. In this screen, if more than one USB drive is attached to your Mac, you need to select the right USB drive that you would like to use for this job. Be sure to select the right one as the drive will be formatted during the process. Click Continue button. Step 5: You’ll see The drive will be erased warning dialog box. Click Continue button to format the drive and begin transferring ISO file contents to the USB drive. If you have chosen Download the latest Windows support software from Apple option in Step 3, Boot Camp Assistant will automatically connect to Apple servers and download necessary support drivers. Step 6: Once Boot Camp completes its business, you’ll see “The selected drive has been formatted and the Windows installation image has been copied” message. You can connect this bootable Windows 8.1 USB to a PC or Mac to begin installing Windows 8.1. If you have chosen to download support software from Apple in Step 3, then you will see the following screen. That’s it! How to create macOS Sierra bootable USB from Windows guide might also be of interest to you.