iMac Hard Drive Replacement Not Working

If all your friend needs is basic Internet browsing and email, then try installing Ubuntu onto the iMac. Ubuntu should install easily and should include the necessary Mac drivers by default. Ubuntu will have access to the latest versions of the most common browsers such as Firefox, Chrome, and Vivaldi. Plus Ubuntu has access to at least two email apps one of which is Thunderbird.

Ubuntu should run really well with 4GB of RAM, but Ubuntu may still be able to browse the internet with as little as 2GB or 3GB if only one or two Windows/tabs are open at one time (although 2GB will be pushing it).

I don't recommend the default Ubuntu as it really requires a powerful computer with lots of RAM. However, the other Ubuntu flavors such as Kubuntu, Ubuntu-Mate are very good choices. Other more basic flavors are Lubuntu and Xubuntu, but they may not be as user friendly. All of the Ubuntu flavors use the same underlying core, but utilize a different graphical interface called a Desktop Environment, plus some of the default apps will be different, but you can install more from the Ubuntu repositories. Most of these Ubuntu flavors will use about 200MB to 300MB of RAM when booted to the desktop for the first time.

I haven't personally used Ubuntu in years, but I do use Debian stable (even on Macs) without issue. Ubuntu is built from Debian with some extra modifications. Debian unfortunately takes a little more effort to setup initially due to Debian's philosophy on non-free drivers and firmware. Since you don't have any OS on the drive now, you really don't have anything to lose in trying one of the Ubuntu flavors.

If the Ubuntu flavors don't have all the necessary drivers installed by default and you find it to difficult to determine what is needed, then try Linux Mint which is based on Ubuntu. It is a very nice looking system, although I personally don't like some of the under the hood modifications they made to Ubuntu.

Use Etcher (Mac/Windows/Linux) to create a bootable Ubuntu USB installer. Option Boot the USB drive and select the orange icon labeled "EFI". If for some reason Ubuntu won't boot after it is installed, then you may need to rename the Ubuntu bootloader located on the hidden ESP/EFI partition. Some older Macs will only boot an unblessed third party OS when the bootloader is located in "/boot/boot.efi" on the hidden EPS/EFI system partition.

If you are not familiar with Linux, then I highly recommend reading the Debian Handbook as it contains a lot of useful information on how to properly manage the system. Most of the Debian information applies to Ubuntu as well. This page contains lots of useful references to Debian base distributions:

https://www.debian.org/doc/user-manuals

The Debian Administrator's Handbook is a great resource even though it hasn't been updated in a few years, but most of the important maintenance & care items should still be valid. You don't need to read everything, but at least understand the basics if you want to support Ubuntu on your friend's iMac. FYI, the biggest difference between Debian and Ubuntu is how the software repositories are distinguished so you will need to consult the Ubuntu documentation regarding the layout of the Ubuntu repositories.

This iMac is extremely old and may not be worth fixing. First purchase a Snow Leopard DVD directly from Apple ($20), here is the link: https://www.apple.com/shop/product/MC573Z/A/mac-os-x-106-snow-leopard?fnode=b9e23076f91bed1b4296f28bced57ec61112b5b98c20e181ae8ca9b37b564af9340b2dfa9fccc0433e8239793f0f70bf7d956722ecd22350bc214733d7b37751e16d872e09b3c9d66cc88c2388350919cccd21008b6236f44c983016f3d7a314

When the drive arrives, boot the computer from the SL DVD and then format the HD you installed. Once the drive has been formatted then simply install Mac OS and you should have a working computer assuming nothing else is wrong. Once the computer boots from the new HD , then update using https://support.apple.com/kb/dl1399?locale=en_US to get to 10.6.8 and then you should be set.

Remember you still will have a VERY old computer that is limited and can't be upgraded beyond 10.11.x however if you just want to use it for surfing the net and e-mail it should be okay.