Installing Mavericks on iMac 24inch 2009

I hope you have the optical recovery discs that came with the Mac.

If you do, use those discs to install the operating system that came with the Mac, which will probably be Mac OS X 10.5.6 (Leopard). Use a modern Mac or PC to download a copy of the .DMG installer file for El Capitan onto a USB flash drive. Transfer the USB flash drive to the old iMac. Copy the .DMG to the iMac, open (mount) it, and run the installer application on the mounted disk image. (Leopard is too old to be able to access modern https sites - and probably will not be able to access this forum, or the linked Apple Support article.)

I believe your iMac can run anything from Mac OS X 10.5.6 (Leopard) through 10.11.6 (El Capitan). As far as I know, Leopard / Snow Leopard / Mavericks are not available for download, but the other compatible versions are.

How to download and install macOS - Apple Support

Ronaldoola wrote:

I don't have a disc plus, I formatted my HDD with the understanding I would just install via internet but it seems I couldn't.

When your Mac was new, there was no such thing as Internet Recovery. Internet Recovery didn't appear until the release of Mac OS X 10.7 (Lion), which Apple distributed mostly in electronic form (although you could buy a very expensive USB flash drive with Lion on it).

Every Mac came with optical restore discs, and it was incumbent on the owner not to lose those discs. It was also good practice to make bootable clone backups (using Carbon Copy Cloner or SuperDuper!).

The only option is to install with a bootable flash which in my case I don't know how.

There is no way for you to create one unless you have a working Mac that's compatible with the version of the OS. Which as far as I can tell, you don't. (If you did have a second Mac and you were comfortable working in Terminal, you could find instructions on how to create an installer in an article linked to by the other Support article.)

You could try using the second Mac to do a "clean install" of El Capitan on an external hard drive … then using that drive to boot the 2009 iMac. If that worked, you could then run the El Capitan installer on the 2009 iMac to put El Capitan onto the internal drive.

Other than that, I don't know what else to suggest.

If you followed the directions that I gave, you did not make a bootable flash drive. You made a flash drive that has a data file (the .DMG disk image containing the El Capitan installer) on it. Even if you restored the disk image (instead of merely copying the .DMG file), you would not have a bootable drive. This particular .DMG is not an image of a bootable drive.

You need to have an operating system running on the old iMac to be able to open (mount) that disk image and to run the installer. This is where optical recovery discs come in. Like my old iMac (27-inch, Late 2009), your iMac (24-inch, Early 2009) predates Internet Recovery, and would have come with optical recovery discs - discs that you would not want to lose under any circumstances.

If you have misplaced those discs, but have a bootable clone backup, you could boot from it.

The link is in the first Support article:

How to download and install macOS - Apple Support

=> Create a bootable installer for macOS - Apple Support

Note that even El Capitan is extremely out of date.

Mozilla Support – Firefox Mac OS X 10.9, 10.10 and 10.11 users move to Extended Support Release

Firefox 78 ESR will run on El Capitan, and is probably a bit more up-to-date than El Capitan's version of Safari – but Firefox 78 ESR itself has not received any critical security update for at least three years.

Mac startup key combinations - Apple Support

I assume that when you restarted the old Mac, you only pressed the Option key?