That's because Mac OS Extended has higher compatibility among different OS versions and the OS installer requires such storage space to load.īy the way, to format a USB drive or partition a volume on a USB drive, go to Disk Utility. Prepare a clean USB flash drive or other secondary volume formatted as Mac OS Extended and with at least 32GB of storage (For macOS Big Sur, 64GB is better). A bootable USB drive should be created on a healthy Mac.Ģ. Here are what you should do before creating a bootable USB drive:ġ. So, you'd better follow this part carefully. The preparations listed below are very important as they decide whether you can create a USB installer successfully or not. If you want to retrieve files from your Mac that won't boot, you can run iBoysoft Data Recovery for Mac in macOS Recovery mode. How to run iBoysoft Data Recovery in macOS Recovery Mode?