Can't format my Macbook, HDD and macUtilities won't load, "blocked" sign after trying to boot usb

Solution 1:

First of all, like Allan mentioned in the comments, your MacOS drive will not show up on startup if you erased it. The reason the 'blocked' sign appears is because the installer files for OS El Capitan are corrupt on your USB.

I also have a white MacBook from 2009 and I encountered the same issues when trying to install macOS Sierra from a bootable USB.

Here's what you can do:

1. Re-create a bootable USB of OS X Yosemite using a Mac computer running MacOS. From my experience, third party applications like Install Disk Creator work better than the Terminal for fully loading the boot files. Note that the USB storage size must be larger than the file size of the installer for the OS you want to install. (8GB worked for me)
2. Run the bootable USB on your MacBook by using the steps you did before. This time, you should be able to get pass the 'blocked' sign.
3. Format the HDD using Disk Utility from the Utilities Menu. Click the 'Erase' tab and set the format of your HDD to 'OS X Extended (Journaled)', then click erase. Your MacBook should then be able recognise the HDD.
4. Install the OS X Yosemite on the HDD using the OS X Installer from the Utilities Menu. You can always update to El Capitan after you install Yosemite. You may have to configure the system time if you run into an error while validating the install.