IMAC not booting correctly.

The "diagnostics" for these older Macs came on one of the supplied CD discs that came with the Mac itself. They are also specific to a Mac model.

Since you were successful booting up this Mac in Safe Mode, indicates that the basic operating system is working, as it disables any third-party device drivers.

With your Mac booted up in Safe Mode, select the Apple icon on the upper left-hand side of the menu bar. Then select "About this Mac." It should provide you with some information that can help us figure out what you have, and what we can try to get it working for you.

Ref:

• Find your Mac model name and serial number - Apple Support
• Identify the model of your Apple device - Apple Support

It sounds like your iMac may be experiencing issues with its startup disk or operating system. Here are some steps you can try to troubleshoot and potentially fix the issue:

1. Check startup disk: The blinking folder icon indicates that the iMac is unable to find a valid startup disk. This could be due to a corrupted macOS installation or a failing hard drive. Start by checking the startup disk in Disk Utility:

a. Boot your iMac into Safe Mode by restarting it and holding down the Shift key until you see the Apple logo.

b. Once in Safe Mode, go to the Apple menu in the top-left corner of the screen and choose "About This Mac."

c. In the About This Mac window, click on the "Storage" tab to see information about your startup disk.

d. If Disk Utility is not available from Safe Mode, restart your iMac normally and immediately hold down the Command (⌘) and R keys to boot into macOS Recovery mode. From there, you can access Disk Utility.

2. Run First Aid: In Disk Utility, select your startup disk and click on the "First Aid" button. This will check and repair any errors on the disk. If Disk Utility reports any issues that it cannot repair, you may need to consider replacing the hard drive.

3. Reinstall macOS: If Disk Utility repairs any disk errors but the issue persists, you may need to reinstall macOS. You can do this from macOS Recovery mode by selecting "Reinstall macOS" from the utilities menu.

4. Check hardware: If the above steps do not resolve the issue, it's possible that there may be a hardware problem with your iMac, such as a failing hard drive or other component. In that case, you may need to take your iMac to a certified Apple service provider for further diagnosis and repair.

Overall, diagnosing and fixing startup issues can be complex, especially on older Mac models. If you're unsure about performing these steps yourself, it's best to seek assistance from a qualified technician or Apple Support.

Great! Thanks for the additional information. The "iMac5,1" would be either a 17- or 20-inch Late 2006 iMac. Just so you are aware, both of these models are considered "obsolete" by Apple Support. That means, Apple doesn't even stock spare parts for these.

The other thing to be aware of is that the latest version of macOS for these is: OS X 10.7.5 (Lion). If we can even get this Mac running again, you will have to ask yourself, what do you actually want to use it for.

For example, if you would like to access web sites over the Internet, chances are this Mac will not be able to as it will not have the correct security TLS certificate level to access secure (think https://) web sites ... including the Apple web sites, like iCloud. The level needed was added, by Apple, with the release of OS X 10.9 Mavericks.

If you just want it to learn about troubleshooting an older Mac, then by all means, it may be worth the effort, and we can certainly try assisting you with this. Just let us know.