Replacing Macbook Pro HDD & Installing Operating System 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.
As far as I understand, you have a running Macbook Pro with an ageing HDD and want to upgrade/replace the current one and also move to Mountain Lion in the process.
I can suggest you the following.


Purchase a USB casing for your new HDD and temporarily convert it to a USB Drive. These are not too expensive and you'll see more benefits in the end.
Connect it to your current system.
Run the installer on your machine and choose to install on the connected USB drive instead of your current system disk.
After the setup is done, remove the HDD from USB casing and replace the system disk.
Use the system disk in the USB casing to have a high capacity backup drive.