On Wed 07 Oct 2026 at 13:54:16 (-0400), Stefan Monnier wrote: > > I noted in an earlier post today, I did get it working on one of my usb > > drives, but it seems sort of a hacky configuration. > > Part of the issue is that the ISO images have historically been created > with a lot of care to make them bootable in a wide variety of contexts.
Agreed. And it's not clear to me whether this ability is a requirement of the OP, as they don't say what they want their USB to do, just that they want to tweak a couple of files involved in booting. > Maybe nowadays things are simpler, but in the past, booting from a CDROM > or a floppy worked a bit differently than booting from an HDD, and then > things got worse with the introduction of GPT partitions and the UEFI > boot protocol, so you had different ways to boot from an HDD with > different tools handling the different options differently (e.g. some > reading only the GPT, other reading only the MBR partition table, > others reading both with different preferences), so it sometimes > required a lot of care to make sure that your Linux kernel, your boot > loader, and your BIOS all agreed about what is where. > > E.g. not sure if current ISOs have both a GPT and an MBR partition > table, but if that's the case, you may have to change both consistently > if you want to add a partition. I wouldn't know how to do this. My understanding is that there are some Debian experts who know how to mangle the USB appropriately. Understandably, the normal tools refuse to perform such delinquent behaviour, as it's like to lead to loss of data in the general case. FYI here's what I observed on the Debian 12.9 netinst: # gdisk -l /dev/sdb GPT fdisk (gdisk) version 1.0.6 Partition table scan: MBR: MBR only BSD: not present APM: not present GPT: present Found valid MBR and GPT. Which do you want to use? 1 - MBR 2 - GPT 3 - Create blank GPT Your answer: 1 Disk /dev/sdb: 7825408 sectors, 3.7 GiB Model: USB DISK Sector size (logical/physical): 512/512 bytes Disk identifier (GUID): 0DC5ADF0-2D26-49C0-9280-6D207B4602A4 Partition table holds up to 128 entries Main partition table begins at sector 2 and ends at sector 33 First usable sector is 34, last usable sector is 7825374 Partitions will be aligned on 4-sector boundaries Total free space is 7806301 sectors (3.7 GiB) Number Start (sector) End (sector) Size Code Name 2 4524 # gdisk -l /dev/sdb GPT fdisk (gdisk) version 1.0.6 Partition table scan: MBR: MBR only BSD: not present APM: not present GPT: present Found valid MBR and GPT. Which do you want to use? 1 - MBR 2 - GPT 3 - Create blank GPT Your answer: 2 Using GPT and creating fresh protective MBR. Warning! Main partition table overlaps the first partition by 64 blocks! You will need to delete this partition or resize it in another utility. Disk /dev/sdb: 7825408 sectors, 3.7 GiB Model: USB DISK Sector size (logical/physical): 512/512 bytes Disk identifier (GUID): D5680A90-A675-40F1-9746-D1817EEF0B87 Partition table holds up to 208 entries Main partition table begins at sector 12 and ends at sector 63 First usable sector is 64, last usable sector is 1294282 Partitions will be aligned on 4-sector boundaries Total free space is 3 sectors (1.5 KiB) Number Start (sector) End (sector) Size Code Name 2 4524 23563 9.3 MiB 0700 ISOHybrid1 # fdisk -l -o Device,Boot,Start,End,Sectors,Size,Id,Attrs,Type /dev/sdb Disk /dev/sdb: 3.73 GiB, 4006608896 bytes, 7825408 sectors Disk model: USB DISK Units: sectors of 1 * 512 = 512 bytes Sector size (logical/physical): 512 bytes / 512 bytes I/O size (minimum/optimal): 512 bytes / 512 bytes Disklabel type: dos Disk identifier: 0x28ebc189 Device Boot Start End Sectors Size Id Attrs Type /dev/sdb1 * 0 1294335 1294336 632M 0 80 Empty /dev/sdb2 4524 23563 19040 9.3M ef EFI (FAT-12/16/32) # As an aside, one used to be able to buy USB sticks that had a "CD" at the start of the drive, followed by a partition of choice. One could set the boundary between the two parts for different sizes. I have one where I've set the CD size to zero. Because of the extra complication, I avoid using it in consumer devices like the car. It's branded SanDisk U3 SMART cruzer 4GB. I don't know whether they're still available on, say, ebay. Cheers, David.
