Installing on Ubuntu 21.04

Ok if either dd under Linux or win32diskimager under windows does not produce a bootable usb drive there are a couple of things.

What I can confirm is that the Linuxcnc ISOs downloaded from the Linuxcnc site are good and do work, Andy puts a lot of work ensuring that before releasing them into the wild. He's a good bloke like that.

Number One & this is important.
If the md5sum of the downloaded image does not match the md5sum file you can download, you will have to try downloading the ISO again. You will have to use google on how to perform this task on your current operating system.

Number 2:
You may have a dodgy usb stick, try another one.

Number 3:
I have seen this personally on rare occasions, some BIOSes have trouble booting from USB sticks larger than 4GB, very rare but worth a mention since you haven't given any machine specific info. Maybe not an issue with you.

Number 4:
I'm not sure how well stretch works with UEFI, but I can confirm the recent Buster ISO does.

I am a little confused as you say the computer shows no bootable devices, yet you also mention that the installer loops back to detecting the cdrom. To me that doesn't make sense, tho I am thinking something has been lost in communication.

If all else fails, sometimes a couple of pictures during the boot\install process helps. As the installer is a standard debian installer you can search whether others with the same system as yours have had an issues.

Finally you can confirm your system is 64 bit capable. Just so we can be on a level playing field.
One thing you can not do, I've seen others trying to do this with Ubuntu and others, is install the system onto the USB stick they are booting from.
The disk you install from and the disk you install to must be physically separate devices. I'm not assuming this is your case but just putting it out there.
I've done things way way more sillier myself. Like taking the fork springs out of my bike without supporting the front of the bike.......No fun when you are 19 and trying to pick up the best part of 220kg on your own.

How old is that PC ???

tommylight wrote: How old is that PC ???

BIOS says 2011. I have had it since 2013.
Intel Atom D525MW motherboard, 1.8 GHz, dual core, 4GB RAM, 60GB SATA SSD, onboard graphics being used

Looking for something better with a PP just haven't come up with it yet.
This one has run LinuxCNC 2.7.xx on Debian since I got it with no issues.

If that is the case, i would stick with Wheezy and just use buildbot to point LinuxCNC to 2.8 instead of 2.7.
Not sure if that would work since Wheezy uses RTAI kernel while the rest uses Preempt-RT kernel, but i am sure there is a version of 2.8 for RTAI ... ..
linuxcnc.org/docs/2.8/html/getting-start...#cha:Installing-RTAI
Or download the Linux Mint 19.3 XCFE edition and see if that boots, then use the above howto to install RTAI and LinuxCNC. Might require some dependencies.

I am sure I have seen a mention of this problem before, and a fix.
This was not specific to the LinuxCNC installer, but was a problem with the generic install.

The advice to plug / unplug the USB while the system is scanning for a CD sounds about right.

Do you mean unplug/replug the USB CD drive, or unplug the USB flash drive?