All bootable devices failed secure boot verification

Did you accidentally switch from UEFI to Legacy boot mode (or vice versa) while fiddling with Secure Boot.  Some operating systems can install in either Legacy or UEFI mode, but after they're installed, they can only be booted if the system is set up in the mode they're expecting.

No, it's still in UEFI mode

Ok, just to confirm, the current message you’re seeing is that no bootable devices are found, correct? Not the message about all boot devices having failed secure boot validation? That’s what I gather from your first post, but typically people don’t name their topics with an error they’ve already solved, so I wanted to check.

Try pressing F12 to bring up the one-time boot menu and manually select the boot device you want. If this works but regular booting doesn’t, you probably just need to add the Bootx64.efi file on your hard drive’s EFI partition to the boot order. If you can’t boot even using the F12 method, it suggests either some damage to the data on the drive or possibly a hardware issue with the drive.

Hi again, when The problem first started the message I got was All bootable devices failed secure boot verification. Then I disabled Secure Boot and from then the message is No bootable devices found.

I've done as you suggested and pressed F12 and not knowing what boot device I want, I ran the diagnostics, which is why this has taken so long to get back to you. All tests passed with a validation code 79886

The other things on the F12 screen are;

Boot mode is set to UEFI; Secure Boot: Off

UEFI: Toshiba MQO1ABD1OO, Partition 1. (Actually I can't make out if the zero's are numbers or letters)

Other Options:

BIOS setup

BiOS flash update

Diagnostics

Change Boot Mode Settings

None of this makes any sense to me. Really hope you can help. It's only a couple of months old and I can't bear the thought of having to send it back, and be without for a few weeks.

Thanks a lot for responding so far

Sh...

That Toshiba device is your internal drive, and if the system arrived in UEFI mode and you never changed that, then trying to boot from that device in UEFI mode is appropriate.  You can go to Change Boot Mode Settings and switch it to Legacy, and then at the F12 screen you'll see the hard drive listed as bootable in Legacy mode, but I don't think that's going to fix anything if the system shipped in UEFI mode.  If the drive passed diagnostics and is still giving you that error, it's possible that a Windows update that went wrong or even a piece of malware has corrupted some data on the drive that's required to boot the system. In that case it's possible that more advanced recovery tools can just fix boot problem, but that's beyond the scope of a few forum posts.  The typical course of action at this point would be to just reinstall Windows.  Some Dell systems have a factory recovery image stored on the drive in another partition that you can boot from using a special key combination, and with other systems you download a large file from support.dell.com that helps you create a bootable USB drive.  Then you connect that drive to this system and use the F12 menu to boot from that and perform the image restore.  Note that this will wipe any existing data on the system, so hopefully you backed up your data.  If you didn't, once again there tools that allow you to boot the system into a Windows environment that will allow you to browse even an unbootable drive and copy data off of it (assuming the data is in fact still there), but that too is beyond the scope of a single forum post.

Sorry for the trouble!  Fyi whenever you get a replacement system, you may want to consider doing at least periodic image backups of your system.  Macrium Reflect is an excellent application for this purpose, and they have both Free and paid versions, but even the Free version is very capable.  Image backups (plus the bootable Rescue Media that Reflect helps you create) would allow you to quickly roll back your entire system to its state at the time of the backup, so as long as you don't have a hardware issue like a dead hard drive or motherboard, you can recover from pretty much anything.  Some people use daily image backups as their primary method of keeping their general data backed up, while others only capture image backups occasionally and choose to use other tools to back up their "regular" data more frequently.

Just for interest I tried to switch to Legacy but it refused, so I guess that confirms it came in UEFI mode.

Thank you very much for your time and comments. Having given it some thought I've decided it is all getting a bit too tricky for my abilities so I've been in touch with the Dell warranty technician who appeared to be equally out of his depth!

He told me to get in touch with Currys/PCworld, from whom I bought it two months ago. This is very disappointing but thank you so much for trying to help me fix it. I've learned some things.

Sh...

I had never even heard of image backups but I do feel motivated to do as you suggest. Don't ever want to be in this position again. Cheers