External DVD playing disks in green only

That would take more testing on your part. Since things work as expected with the built-in DVD Player in a new account, then we know for certain something installed in your normal user account is causing the issue.

Please post the EtreCheck report I requested earlier. That will show us all loaded/running processes. From there, we may be able to determine what's interfering with the DVD Player.

Great to hear so far my friend!:)

Might take awhile to narrow it down further, just guessing here...

Quit DVD Player & try VLC Player...

https://www.videolan.org/vlc/download-macosx.html

If you wish to try DVD Player fix, quit Sys Prefs, delete these files...

~Library/Preferences/com.apple.iDVD.plist

~/Library/Preferences/com.apple.DVDPlayer.plist

~ tilde indicates  your user library folder

Apple hid the Users' Library folders...

Method 1:

1. From the Finder, select the Go menu at top of the screen, and choose Go to Folder.
2. In the window that opens, enter ~/Library, and click Go.

Or…

 Use the Command-Shift-period

Get Onyx & deep clean all the caches & restart...

https://www.titanium-software.fr/en/onyx.html

I think BDAqua wants to find out if something in your normal user account is causing the problem. A new account will have all default settings, which would rule out third party software that may be causing the problem. Though even a new account can be affected by launch daemons that are in the root Library folder.

It may be hard to track down, as the port you're plugging the drives into may be bad. The video hardware may have a fault. The video drivers may be damaged, which only a reinstall of the OS may fix.

That was just to get rid of a kernel extension Seagate should never have had the user install in the first place. All it accomplishes is adding more complexity to file management. That is, it tries to do what the OS is already designed to do on its own. Copy, move, delete.

But, there was a very outside chance it could be causing the green issue. Poorly written kernel extensions can cause a lot of weird, and seemingly unrelated things to happen.

I don’t see anything that would trigger green video.

I’d first try playing video in a second and newly-created login, to see if there’s some weird setting here.

I’d then reinstall, on the off chance that there’s a storage corruption lurking somewhere here.

I’d leave off MalwareBytes, the Seagate add-on, and everything else you don’t absolutely need, as this 8 GB / HDD iMac doesn’t have nearly the necessary resources and capacity for all of what is loaded here.

There are two ways to do that reinstall. One is to reinstall macOS atop macOS, which should not disturb what is installed. Have backups anyway. The second is to back up, wipe and reset and install the newest supported macOS for this iMac, and use Setup Assistant or Migration Assistant to restore your files from your backup.

It’s also quite possible that the graphics hardware in this iMac is simply failing.

Apple Diagnostics won’t spot transient errors and won’t spot all hardware errors, but I’d still run those anyway.

No, that's already been ruled out by your own testing.

The external DVD plays fine under Guest-User.

Which means something in your main account is causing the problem. This could be any number of third party add-ons.

Download and run EtreCheck. You do not need the paid version. It's an app written by fellow forum member, etresoft, and is intended, more than anything else, to provide information about your system so other users don't have to play 20 Questions to get this same information. All personal information is automatically redacted.

When the app is done running, click the Share Report button at the top, and then Copy report so it gets sent to the clipboard. Next, start a new reply here, click on the Additional text icon and paste it there. The reports are almost always too long to paste into the main reply window.

I doubt a virus.

To force eject a CD/DVD hold mouse bbutton down at booting up... might only work with wired USB Mouse.

On your Mac, To find out if it's system wide or user specific, try this...

Open System Preferences>Users & Groups, unlock the lock, click on the little plus icon, make a new admin account, log out & into the new account.

Does it work in the new account?