Unable to boot Inspiron 24 5420 from USB to install Chrome OS Flex

Turn off Secure Boot in BIOS settings to see if it may help.

@Chino de Oro​

Disabling secure boot and fast boot were my first two attempts. 

Check storage controller in BIOS settings and turn off RAID see if it may help.

Create a Windows installation USB from Windows Creation Tool for testing.  If it can boot into Windows installation screen, then the issue is with the Chrome OS installation drive.

@Chino de Oro​ 

I set the BIOS to "AHCI/NVMe" instead of "RAID On"but that did not work. The USB can be selected when pressing F12 or via the Boot Configuration, but as soon as the Dell Splash shows up, it freezes and all power is cut (USB stops blinking).

I'll try making a Windows Installation USB to test it.

Also, just thinking, both of the USB sticks I've tried were 8GB 2.0, but the USB slots on the computer are 3.0.

Could that be an issue?

No, USB 2.0 drives should work fine in USB 3 ports.  Certain boot parameters may caused the issue but it won't be confirm until you have complete troubleshooting.  Let see if a Windows 10/11 installation drive can load.  You can also use a different brand of USB for testing.

For purpose of system warranty and support, remove the system original boot drive for safekeeping and use a new drive for Chrome OS 

@Chino de Oro​ The windows boot worked. So I suppose it's a Chrome OS issue.

Any suggestions then? I can try making a dd boot drive like this: https://support.google.com/chromeosflex/answer/11541904?hl=en&sjid=8436212406713170201-NA#:~:text=from%20the%20device.-,Download%20from%20Google,-For%20admins%20who

I need to use my Mac at home to make it.

Now that the issue is identified, just create the installation USB again.  You should be able to create the installer with Chromebook Recovery Utility.

Use Chromebook Recovery Utility

Step 1: Install Chromebook Recovery Utility

1. On your Windows, or Mac device, open Chrome browser .
2. Go to the Chrome web store.
3. At the top right, click Add to Chrome.
4. When prompted, click Add extension.

The Chromebook Recovery Utility is now an extension in your Chrome browser.

Step 2: Make sure Chrome Recovery Utility extension is turned on

1. At the top right of your Chrome browser window, click Extensions Manage extensions.
2. Next to Chromebook Recovery Utility, toggle the switch to the right.
   Optional: You can also click Details and toggle the switch to the right.

After you turn on the Chromebook Recovery Extension, in the extension panel in your browser, click on the extension to open the pop-up.

Step 3: Build the USB installer

1. In Chrome browser, launch the Chrome Recovery Utility extension.
2. Click Get started.
3. Click Select a model from a list.
4. For Select a manufacturer, find and click Google ChromeOS Flex.
5. For Select a product, find and click ChromeOS Flex.
6. Click Continue.
7. When prompted, insert your USB drive.
8. From the dropdown menu, select the USB drive.
9. Click Continue.
10. Click Create now.
    Note: During the process, it is normal for Chrome Recovery Utility to show unusual percentages.
11. When you get a message that your recovery media is ready, remove your USB drive from the device.

@Chino de Oro​ yes i have done that many times. The chrome installer crashes when trying to use 

@Chino de Oro Just an update. I got a USB 3.1 and I made a dd USB boot using my Mac. It does the exact same thing. When I select the USB to boot from, the Dell splash screen shows up and it freezes immediately

Verify if your AIO system is still on certified compatibility list https://support.google.com/chromeosflex/answer/11513094?hl=en#zippy=%2Cdell

Since the Windows installation USB could launch without issue on your AIO, focus on testing the Chrome installation USB on another device to ensure that it was properly made.

@Chino de Oro​ It's not a model on the certified compatability list. I've installed Chrome OS Flex on dozens of computers and none of them have been on that compatability list.

The installer works fine on every other computer. The regular one, the one I made using the recovery file, and the dd USB I just made. All work on every other computer except this AIO Inspiron 24.

Disable Secure boot and turning off RAID were suggested at the beginning.  Not much else that I could think of causing the installation to hang.

@Chino de Oro​ Can confirm that disabling Secure Boot and switching from RAID to AHCI/NVMe did not work. Still stuck/frozen on the Dell spash screen.

@FishtownZ​ did it end up working for you? I have a 10 year old Inspiron 3521 and have the exact same issue. Can not boot past Dell splash screen -