Long story short I am in desperate need of help figuring out how to prepare for and replace my Macbook Pro (Mid 2012) hard drive. An SSD is not an option, to clarify, and I've already purchased a hard drive for the computer, so that step is complete.
I am having some issues creating a bootable flash drive with the operating system. I am currently running Mountain Lion on the computer, but obviously only have an installer for the most current operating system, which is High Sierra. I cannot seem to get the High Sierra on a bootable flash since I'm not able to run this update on my computer with a failing hard drive/S.M.A.R.T. Issues. So firstly, I need to know if I can somehow download the latest up to date operating system on the flash drive. If not, can I purchase it somehow on a disc or something from apple and reinstall it on the new hard drive? I want to simplify this as much as I can.