Anyhow, it shows that the second partition on the /dev/sde drive holds an iso9660 filesystem. Presumably the Ventoy iso. (It seems the UEFI bios reorders the drive partitions so that the EFI partition counts as the first one, not mounted)

It's the "load modules" step that requires the media to be mounted for the Chimaera ISO. Though if the network works with the initial modules you can complete an installation without access to the media provided you don't need to set up the target partition as, say, ext4 as part of that installation. The Daedalus ISO boot up requires access to the media at an earlier point and it fails when that media is missing.

One could imagine that the second /dev/sde partition indeed would be the target iso file in some virtual loop-back mounting done by Ventoy software "behind" the kernel, but then this would be the same for Daedalus.

You gave me the Ventoy version (96) on email so I can experiment with it myself.

As I looked into Devuan 5.0.1 ISOs I saw that I had it wrong: that ISO does not consider any Ventoy labelled partition. I then realized I have been referring to an unpublished Daedalus 5.1.0 that was handed out specifically to someone some while ago.

I remember reading that thread too.

Ventoy stick booted, Secure Boot off, Chimaera netinstall started in efi mode.

When the partitioning is started and waits for the method to partition, Alt-F2 to the 2nd console, mount tells me:
rootfs, tmpfs, sysfs, devtmpfs, devpts, and then the interesting things:
/dev/sde2 on /cdrom type iso9660 (ro,relatime,nojoliet,check=s,map=n,blocksize=2048,iocharset=utf8) 
none on /sys/firmware/efi/efivars type efivarfs (rw,relatime)

As mentioned in the private mail: I only had problems when skipping the topic "Loading additional modules" in the expert installer mode. I have got full choice, I am using manual partitioning and I am encrypting all my installations (with very few exceptions in a VM).

Hope that helps.

But what does "mount" say (when chimaera installer at partitioning)? And does that partitioner offer a large number of partition types? Maybe I'm wrong there too?

/dev/sdg1: LABEL="Ventoy" UUID="D7EA-AE3B" BLOCK_SIZE="512" TYPE="exfat" PTTYPE="dos" PARTUUID="f078671f-01"
/dev/sdg2: SEC_TYPE="msdos" LABEL_FATBOOT="VTOYEFI" LABEL="VTOYEFI" UUID="B2C8-40D2" BLOCK_SIZE="512" TYPE="vfat" PARTUUID="f078671f-02"

I have not modified anything. Just used the Ventoy installer, let it do the job, nothing tweaked. Except for added a Tools folder to the exFAT partition for additional SW and files.

If I remember right, the partitioning step needs modules from the media to set up ext4 partitions.

Could you when the partitioning step starts make a detour by using Ctrl-Alt-F2 to enter shell, and there type "mount" to see what is mounted... what is the output of that?

EDIT: also, what does blkid print for /dev/sdg1 ... Have you tried labelling that "Ventoy" ?
there may be an exfatlabel utility for that or tune.exfat.

my Ventoy stick is shown as this:

# fdisk -l /dev/sdg
Disk /dev/sdg: 114,61 GiB, 123060879360 bytes, 240353280 sectors
Disk model:  SanDisk 3.2Gen1
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: 0xf078671f

Device     Boot     Start       End   Sectors   Size Id Type
/dev/sdg1  *         2048 240287743 240285696 114,6G  7 HPFS/NTFS/exFAT
/dev/sdg2       240287744 240353279     65536    32M ef EFI (FAT-12/16/32)

I can boot the older Devuan releases as well in efi as in legacy mode and install them from the boot of the iso, the first language selection dialogue until the final eject media, they are fully installed afterwards. No limits.

I know it runs and so does daedalus, but they differ in when they need the media. Both of them have the problem that when they need to mount the media, they fail.

EDIT: Though, the Daedalus boot preamble includes logic to look for the ISO as a file in a partition of type exfat and labelled "Ventoy".

So if the Ventoy partition would be labelled "Ventoy" and is of type exfat, then the Daedalus preamble may find the file and loop-back mount that for the installer (as well as for finding the actual installer initrd in the first place). And in that case installation is made possible. This logic is not present in the chimaera iso.

Daedalus is the only release that fails on Ventoy. ASCII, Beowulf and Chimaera work fine.

I am surprised to hear that! Afaik they all need to mount the media, and ventoy doesn't offer it as mountable. Perhaps you could provide me with a such loaded USB with, say, chimaera, to investigate?

EDIT: the installer may well start and bring up a dialog, but it fails after a couple of dialogs when it comes to the step that needs to mount the media. That step essentially uses blkid to find the mountable devices. When using a ventoy boot, the ISO is merely a file somewhere in the first partition.

On the other hand, Ventoy seems to be sort of dead, according to their web page there will be no further version past 0.99.

Hi, sorry for the delay. I was curious about this statement, but it appears that there are some updates recently according to the Ventoy site: https://ventoy.net/en/doc_news.html

Though I do wish I could find a way to make it use the onboard isolinux in an iso instead of having to add syslinux on top of that.

Mintstick works perfectly with Daedalus too.

Although I'd still like to ask here, since there's an ongoing conversation about it, is there an easy way to tell when an ISO will work on Ventoy or does the project need to specifically mention this? The reason I ask is because in the past I had similar problems with other ISO files, but I wasn't sure if they were caused because of this or just a bad USB stick.

No, there isn't an easy way. As far as I am concerned, Daedalus is the only release that fails on Ventoy. ASCII, Beowulf and Chimaera work fine. Same for all Debians I have tried, Arch Linux, AcroLinux, Mint > 20.1, LMDE 5 and 6, ...

On the other hand, Ventoy seems to be sort of dead, according to their web page there will be no further version past 0.99.

Although I'd still like to ask here, since there's an ongoing conversation about it, is there an easy way to tell when an ISO will work on Ventoy or does the project need to specifically mention this? The reason I ask is because in the past I had similar problems with other ISO files, but I wasn't sure if they were caused because of this or just a bad USB stick.