I found this worked perfectly (from the terminal):
diskutil eraseVolume ExFAT MyName diskX
You'll need to change diskX to whatever the number is for your drive. You can find that out in disk utility, select the drive, click info, and look under 'BSD device node'
Answer to question 2
No, not having Bootcamp does not prevent you from formatting the Mac's internal drive. In fact, the great majority of Mac owners do not have Bootcamp installed. Bootcamp is simply a tool released by Apple that allows Mac owners to install and boot their Macs natively into a Microsoft Windows installation. This happens alongside an existing macOS installation, but does not allow you to run Windows and macOS concurrently. It is a good option if you need to run Windows programs that are CPU-intensive in nature and don't lend themselves to working well in a virtual machine (such as ones set up with Parallels or VMWare Fusion).
Answer to question 1
I've looked at the photos I asked you to post and the second option after booting up from the USB should be Install OS X, not Reinstall OS X. This implies to me that you're USB is not in fact bootable because the 'Reinstall' option would normally appear when booting from the Recovery partition.
Can you try choosing the Reinstall OS X option instead to see whether you're able to successfully install a version of OS X. Let us know how you go.
Also, the photo of the error you get provides a lot more information. Basically the reason you can't erase the drive is because it can't be unmounted. This tells us a lot more about what may be going on, so if choosing the Reinstall OS X option gives you no joy then we've got more info to work with.
[EDIT]
Since you're unable to unmount the internal drive, and since the various terminal commands haven't worked, I strongly suspect that my earlier suspicion is correct - that your 'bootable' USB is in fact not bootable. Your question doesn't detail how you created the bootable USB, so let's start again using these steps:
- Recreate your bootable USB. Since you're limited to doing this on
Windows, follow the steps in this article.
- Plug the USB into your Mac.
- Switch the Mac on and immediately press the Option key.
- When you see the choice of drives, select the USB.
- Once the OS X Utilities window appears, select Disk Utility.
- Try to erase your internal drive and see if you're now able to erase the disk. If unsuccessful take note of the error message etc. Regardless, go to the next step.
- Exit Disk Utilities and select Install OS X.
- Follow the steps to install OS X.
If you fail at either (or both) steps 6 and/or 8, please report back with what happens.
Best Answer
You need to boot from another disk and run disk utility and install from that drive.
You cannot boot from a disk that you want to erase or partition or re-install the system on.