I do seem to have some issues getting and upgrading packages, but I assume that's more of a networking issue I can figure out (I don't have ethernet, so I used my phone's internet during the installing).

Thanks for all the help, everyone!

I don't know if this was because of what I did to Ventoy/recopy, or if it would have worked before with just the command, and I just gave up after it froze too soon, but it finally worked. I'll continue with the installation and see if everything works. I'll report back if I reach another error or if it finishes with no more problems.

Edit: it then froze when clicked Network Config option. It seems like the freezing was a separate issue from the media detection issue (or maybe it's just that error = freeze at times or something like that)

Edit 2: Yeah, it probably is a separate problem. Doing it again, I easily got past detecting network. Maybe the freezing is just a hardware issue, or maybe an idling issue?

tried again today:
with ventoy-linux (./VentoyGUI.x86_64) the netinstall went through with both MBR and GPT
with ventoy-windows (Ventoy2Disk) the netinstall went through with MBR and NOT with GPT!

if i used GPT (via Ventoy2Disk), receive the same error you posted earliert

so i suggest you try again format with Ventoy and let use partition Style "MBR" which is standard setting for Ventoy.
that should work.

https://ibb.co/CKZ7HcsG

sh /lib/debian-installer-startup.d/S21ventoy-loop

to get your iso loop device set up properly.
Could you run the "expert install" and find out on which step does it freeze?  Are you able to change to tty4 at that point and read any debugging text?

`sh /lib/debian-installer-startup.d/S21ventoy-loop ; losetup -a` gave `/dev/loop0: 0 /mnt/devuan_excalibur_6.1.1_amd64_netinstall.iso` as mentioned.

`mkdir -p /cdrom ; mount -t iso9660 /dev/loop0 /cdrom` gave nothing

Then, continuing installing froze everything (though this happened on the attempt immediately before as well, so likely not because of the above)

I selected GPT when creating my USB with Ventoy

Good luck.

Edit: if you really want to use ventoy, then there is option "partition style" in VentoyMenu > it should be set to "GPT" for coexistance with Win11 i suppose; have checked this just yet, on linux there exist such option. otherwise, with standard-setting "MBR" you run into conflict, most probably. but windows-ventoy might handle it differently. just a warning.

Edit2: just tested this with excalibur.netinstall-iso. with ventoy:

when using standard settings (mbr) > it would fail exactly at the point you described;
when formatting with partition style (gpt) > the netinstall would process, you could proceed.

please disregard the upper portion with "default settings" then; you should set most probably BIOS as described earlier, with SecureBoot (ON or OFF) and if (ON) with "Custom" setting; or the like.

What does this return?

losetup -a

If it returns nothing, try this

sh /lib/debian-installer-startup.d/S21ventoy-loop ; losetup -a

If there's this line: /dev/loop0: 0 /mnt/devuan_excalibur_6.1.1_amd64_netinstall.iso - then try this:

mkdir -p /cdrom ; mount -t iso9660 /dev/loop0 /cdrom

If it mounts correctly, then see if installation will continue.
If there are any errors/info displayed, please report back.

Is it possible that the problem is that the boot order in my BIOS doesn't start from USB? I doubt it, but I'm not too familiar with it to know either way.

Under normal circumstances: no. You have a brandnew efi-only system. To boot from any device connected to your main board you go into the bios and directly select the device or you can use boot-override. This is a nice efi feature: you press a certain key during the boot process, and get the boot menu as result and can select the device to boot from. This key you need to press depends on the mainboard/efi bios manufacturer. On my ASUS mainboard I have to use F8. When I am not mistaken you have a MSI mainboard. Please read the manual, it should be documented in there.

sleep 1
DEV=$(blkid -L Ventoy)
mount -t exfat $DEV /mnt
modprobe loop
losesetup -f /mnt/devuan_excalibur_6.1.1_amd64_netinstall.iso

Is it possible that the problem is that the boot order in my BIOS doesn't start from USB? I doubt it, but I'm not too familiar with it to know either way.

mount: mounting /dev/sda1 on /cdrom failed: Invalid argument
mount: mounting /dev/sda1 on /cdrom failed: Invalid argument
mount: mounting /dev/sda2 on /cdrom failed: Invalid argument
mount: mounting /dev/nvme0n1 on /cdrom failed: Invalid argument

Then:

cdrom-detect: CD-ROM mount failed: device=/dev/nvem0n1 fstype=vfat
cdrom-detect: CD-ROM mount failed: device=/dev/sda fstype=iso9660
cdrom-detect: CD-ROM mount failed: device=/dev/sda fstype=vfat

Also, output of "mkdir -p /cdrom ; mount -t iso9660 /dev/sr0 /cdrom" gives me:

mounting /dev/sr0 on /cdrom failed: no such file or directory.

And "ls /sys/class/block" gives me:

loop0 through loop7, nvme0n1, and sda through sda2

sha256sum
https://get.refracta.org/files/experime … iso.sha256

3eee6699e8a6c4e5cd1209376b7943678bf973cea571732ac96c7ed3d50f18ec  excalibur_nonfree_bpo_7.1.3-20260728_2304.iso

(FYI, I've been unable to get #1 and #3 to boot properly from ventoy, at least in efi mode.  But you're welcome to test, of course.)

If you choose #4 above, it would be helpful to know the debugging output text on tty4 after it fails to detect installation media.
For reference, the output on my test system (with ventoy) says:

cdrom-detect: Searching for Debian installation media...
cdrom-detect: Devices: '/dev/sr0'
cdrom-detect: CD-ROM mount succeeded: device=/dev/sr0 fstype=iso9660
cdrom-detect: Detected CD 'Devuan GNU/Linux 6.1.1 excalibur amd64 - netinstall 20260425'
kernel: ISO 9660 Extensions: Microsoft Joliet Level 3
kernel: ISO 9660 Extensions: RRIP_1991A
cdrom-detect: Detected CD with 'stable' (excalibur) distribution
anna-install: Queueing udeb eject-udeb for later installation

What happens if you try to run, in tty2:

mkdir -p /cdrom ; mount -t iso9660 /dev/sr0 /cdrom

Also the output of this might be helpful:

ls /sys/class/block

(you can press alt + left-or-right arrow to switch between tty windows)

Is there a working Freia netinstall available? On the internet I see a version dated June 30th? Is that good? Haven't tested it yet. Then I would try "Testing".