Problems flashing USB drive on M1 iMac

[Mohs]

Hi, I’m trying to flash a USB drive to install Lubuntu in an old Macbook. I’m using Etcher 1.19 on a M1 iMac running Sonoma. Etcher appears to run and I get the “successfully completed” message at the end but it fact the USB is useless. Regardless of whether it’s initially formatted as exFAT or MacOS extended, the USB disappears (un-mounts) from the desktop during flashing and it does not re-mount when the flashing is “complete.” Using Disk Utility I see a small partition labelled “ESP” in the USB which can be mounted manually. Inside this partition there is an EFI folder but it has only 4 MB inside instead of the entire Lubuntu package which should be over 1 GB. If I put the USB in another Mac, it does not mount. Any thought? Alternatives to Etcher? Thanks for your help.