Why can't newer Mac hardware boot DBAN? I have a DBAN CD )used with an external optical drive) which I have used in the past on multiple old Macs. I tried to use it on a 2018 Macbook Pro and DBAN could not be booted. Specifically, I tried holding down the "c" key and I could hear the CD spin a lot but then the normal OS booted. I tried booting while holding down "Option" but DBAN wasn't in the list of bootable options. I tried booting into Recovery and selecting the DBAN CD as the startup disk but I can't choose it (I see the DBAN CD but when I choose it I receive a message that the startup disk can't be used because "the bless tool was unable to set the current boot disk"). Again, if I open Terminal in Recovery Mode and try to bless the CD then it fails.

I wanted to know if this was a problem unique to one Mac so I tested the same CD in an old Macbook Pro (I think 2014 or 2015) and a different 2018 Macbook Pro. The CD is bootable normally in the old one and not in that second new one I tested.

Did Apple make a firmware change in newer systems that could have this effect?
T2 chip in newer Macs needs to recognize the boot media to allow it to start from that disk. You might be able to tell the system to use the drive as a startup volume if you can log in first with sufficient privileges (Preferences --> Startup Disk). Workaround? Boot to recovery partition, open Disk Utility, and do a secure wipe of the drive, using highest stringency options. Accomplishes the same thing (7 pass U.S. Department of Defense 5220-22-M compliant wipe) without the need to run DBAN. Also leaves the machine in a recoverable state so that the next user can install from the recovery partition, which is far faster than Internet Recovery (and in some cases, DBAN wipes the recovery partition, which makes reinstalling incredibly painful depending on the machine - I spent several days repairing an old Air after our IT folks used a DBAN equivalent to wipe it - even worse, this happened AFTER I had already run a full wipe from disk utility - the DBAN wipe wasn't even needed at that point!)