Installer fails to find add-ons from directory

SUSE Linux Enterprise Server SLES Install-Boot
1

We are trying to use AutoYaST to install SLES15SP1 from an ISO on a local hard drive, using an alternate entry to the GRUB2 menu. The ISO is at the root of /dev/sdb1, and we have baked into the ISO three repo directories (and one including pre- and post-scripts). So, the unpacked ISO file structure looks like this:

ISO
├── autoinst.xml
├── boot
│   └── x86_64
│       ├── grub2-efi
│       │   └── themes
│       │       └── SLE
│       │           └── icons
│       └── loader
├── docu
├── EFI
│   └── BOOT
│       └── locale
├── media.1
├── Module-Basesystem
│   ├── media.1
│   ├── noarch
│   ├── repodata
│   └── x86_64
├── Module-Server-Applications
│   ├── media.1
│   ├── noarch
│   ├── repodata
│   └── x86_64
├── noarch
├── Product-HA
│   ├── media.1
│   ├── noarch
│   ├── repodata
│   └── x86_64
├── repodata
├── scripts
└── x86_64

The ISO boots fine and the installer begins, but when it gets to Adding the Repository… → Probe Source Type, it says,

[QUOTE]Cannot access installation media
iso:/?iso=MyIso.iso&url=hd:/?device=/dev/sdb1
(Medium 1).
Check whether the server is accessible.[/QUOTE]

I’m not actually sure what to make of the error, since it’s obvious the installation media is available at that url, else we wouldn’t be able to boot from it. Our autoinst.xml includes the following:

 <add-on>
   <add_on_products config:type="list">
    <listentry>
     <media_url>cd:///</media_url>
     <product_dir>/Module-Basesystem</product_dir>
     <product>sle-module-basesystem</product>
    </listentry>
    <listentry>
     <media_url>cd:///</media_url>
     <product_dir>/Product-HA</product_dir>
     <product>sle-product-ha</product>
    </listentry>
    <listentry>
     <media_url>cd:///</media_url>
     <product_dir>/Module-Server-Applications</product_dir>
     <product>sle-module-server-applications</product>
    </listentry>
   </add_on_products>
  </add-on>

… but that media_url isn’t what the installer’s barking about.

Can anyone here help me figure out how to point the installer to those directories? I’m happy to answer any other questions as needed. Thanks in advance.