Can I boot a 2015 iMac Mojave OS into FreeDOS from a properly prepared DVD or USB drive?

In a bit of a jam: My 2015 iMac has just come back from repair for a broken Fusion Drive. Unfortunately, the replacement HDD needs a firmware update, but this was ignored by the repair shop. I'd rather try to resolve this myself than fight it out through the credit card company. Due to the difficulties in accessing iMac's internal hardware, the firmware upgrade must be done *in situ*. The [firmware upgrade is available from Seagate](https://www.seagate.com/support/kb/barracuda-1tbdisk-platform-firmware-update-223651en/) , but it's packaged with FreeDOS in a "bootable" ISO format. Seagate even supplied some [instructions for preparing a bootable CD/DVD](https://www.seagate.com/support/kb/how-to-burn-an-iso-image-onto-a-cddvd-rom-201431en/#MacOS) , but it seems these instructions are for an older version of macOS. Their instructions didn't work on my 10.14 Mojave system, but I found another approach that seems to have worked: | Finder Window | Burn Disc | | -------- | -------------- | | | ![enter image description here ][2] | Unfortunately, there's a bigger issue: While the ISO was burned to the DVD, this DVD isn't recognized by the system as a 'Startup Disk'; it isn't presented as an option after a Restart when the option key is depressed. I have also [tried making a bootable USB drive](https://help.ubuntu.com/community/How%20to%20install%20Ubuntu%20on%20MacBook%20using%20USB%20Stick?_ga=2.190828974.429803735.1668245635-2129123856.1668245635) using the "Other Procedure" listed there, but the USB drive wasn't presented as an option either. I tried using balenaEtcher to create a bootable USB using the .dmg file created with hdiutil convert -format UDRW -o ~/path/to/target.img ~/path/to/Barracuda-ALL-GRCC4H.iso. This also failed, but Etcher gave a potentially useful error message shown in the screenshot below. #### How do I boot my iMac into FreeDOS so I can install the firmware upgrade on my HDD?