Assuming Vista or 7 (this should work on XP, 8, 8.1, and 10, as well) and that the disk is not showing up under My Computer at all:. Connect your disk. Run cmd as an Administrator. Run diskpart.exe. If you need help in this program. list disk.
Find the disk that corresponds to your USB disk. Select disk n where n is the number of the disk.
Insert your USB drive (you'll need one that's 8GB or larger) and open Disk Utility. Select your drive in the sidebar and go to the Erase tab. Format the drive as 'Mac OS Extended (Journaled)'. Take a 16GB or larger USB flash drive to create a bootable Sierra installer. This flash drive is only needed for a clean install on your Mac’s startup drive. But if you plan to clean install on a non-startup drive (you have and extra HDD or SSD on your Mac), you don't need the USB.
Confirm that you're using the right disk with detail disk. clean (Warning: This command erases the disk's partition information. Any data on the disk will no longer be accessible.). create partition primary. No size is needed if you want to use the whole disk. active.
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Marks the partition as potentially bootable. format fs=fat32 quick. You can choose NTFS or exFAT instead of FAT32 if you want. (Note: Windows 10 limits FAT32 to 4GB. I recommend using exFAT instead, which is essentially a newer version of the same format.). assign.
Assigns the disk a drive letter. exit to quit. If you're still having problems with the disk after trying this, you might try omitting the quick from step 9 to do a full format. That will take a very long time and usually isn't necessary, but may help uncover physical damage to the disk. In some cases you might need to re-initialize the disk. As above, this will destroy the data on the disk (or, rather, your ability to access it). To do that from diskpart.exe:.
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list disk. select disk n. attributes disk clear readonly.
This command will unset the read-only flag on the disk. You can see the current attributes with attributes disk or detail disk. online disk.
Sets the device status to online if it's been disabled. convert mbr. Converts the disk to MBR format, which will work just fine for most disks. If you've got a USB disk larger than 2 terabytes, however, you'll want to use convert gpt instead. I'm not sure if the above commands are all identical on older OSs (XP).
After, follow these steps to install macOS:. Choose Reinstall macOS (or Reinstall OS X) from the Utilities window. Click Continue, then follow the onscreen instructions to choose your disk and begin installation. If the installer asks to unlock your disk, enter the password you use to log in to your Mac.
If it doesn't see your disk, or it says that it can't install on your computer or volume, you might need to. Please allow installation to complete without putting your Mac to sleep or closing its lid. During installation, your Mac might restart and show a progress bar several times, and the screen might be empty for minutes at a time. If your Mac restarts to a setup assistant, but you're selling or giving it away, press Command-Q to quit the assistant without completing setup.
Then click Shut Down. When the new owner starts up the Mac, they can use their own information to complete setup.
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