Unknown Error occurred when installing Veeam Software Appliance
In this guide we shall discuss how to fix “Unknown Error occurred when installing Veeam Software Appliance”. Veeam Backup & Replication v13 also referred to Veeam Software Appliance v13 introduces a fully 64-bit architecture, delivering significant improvements in performance, scalability, and memory efficiency. Please, see how to setup Veeam Software Appliance v13, Deep Dive into Protecting AWS EC2, RDS Instances and VPC, and How to Configure Object First OOTBI Appliance,
This version enables a completely Linux-based backup infrastructure. Therefore, eliminating the need for any Windows components, as all core roles and services are now fully supported on Linux.
Another error you can run into during the installation is the “Appliance Installation Stuck on “Configuring addons“. Also, see How to fix DISM Error 87: The package is unknown, and How to fix Unknown Hard Error on Windows Server and Windows 10.
Why was the Unknown Error occurred when installing VSA Prompted?
From the very first solution (workaround) to fix this issue. I found out that the ISO was not correctly copied to the Proxmox storage (NFS, local etc).
Please, see Cannot find the computer: Fix the following error occurred while using Kerberos authentication, and “How to Fix “Unknown hard error” on Windows Server and Windows 10“.
Resolve Unknown Error occurred
I decided to download a new Veeam Backup and Replication ISO and verified the checksum. This means, downloading the ISO again ensures you have a clean, complete copy (in case the first download was corrupted).
Verifying the checksum is the proper method to confirm that your Veeam Backup & Replication ISO is genuine and hasn’t been modified. Therefore, I removed the old copy of ISO uploaded as shown below.
The newly downloaded copy is now uploaded and I stayed glued to this screen to ensure the upload completed correctly. Do not not stop this process for any reason. Else, you will have some unwanted outcomes.
As you can see below, the upload has now completed successfully.
The below status is also okay as shown below.
Please, see How to Resize or Expand Proxmox Hard Drive, how to perform Offline and Online Update for Object First Out-of-the-Box Cluster, and how to add Synology NFS Storage to Proxmox VE for VMs and Backups.
Remove ISO (CD/DVD) or Change Boot Order
If you do not remove the CD, after installing Veeam Software Appliance (VSA). You will be prompted to re-install after VM reboots.
Alternatively, you can uncheck or move the boot order down or remove it as shown below. But when the CD/DVD is removed, the order changes automatically as well.
For other VMs that have the ISO attached for example Veeam Hardened Repository (VHR), and some Windows Server. This is because, some Windows Server and Linux-based appliances may fail to boot once the installation ISO is detached. here is how to Install Windows Server 2025 via iDRAC Virtual Media or PXE.
This occurs when the UEFI boot entry or GRUB loader remains linked to the ISO environment instead of the VM’s virtual disk. To avoid this, recreate the EFI boot entry manually from the Proxmox console (efibootmgr) before detaching” or “Keep the ISO attached but uncheck “boot from CD-ROM” in the boot order” as shown below.
Please, see Fix EFI network timeout on VMware Workstation, and How To Fix “Startup Repair Couldn’t Repair Your PC.
FAQs
Why does my VM only boot when the installation ISO is attached?
his usually happens when the EFI or GRUB bootloader was installed while booted from the ISO. Therefore, causing the system to reference the ISO’s boot path instead of the VM’s virtual disk.
Can I safely detach the ISO after installation?
Yes, but only after confirming that the VM boots directly from its virtual disk. In UEFI-based systems, ensure a valid EFI boot entry exists using efibootmgr -v inside the VM or by checking the “Boot Order” in Proxmox.
Why does Windows Server sometimes fail to boot without the ISO attached?
Because both rely on UEFI initialization during setup. If Proxmox doesn’t write a persistent EFI boot entry to the virtual disk, the ISO remains part of the boot chain.
How can I fix a VM that fails to boot without the ISO?
If your VM only boots with the ISO attached. You can repair the bootloader by booting again with the ISO connected. For Linux, open a recovery shell and reinstall the bootloader using:grub-install /dev/sda or create a new EFI entry with efibootmgr --create. For Windows, use the recovery console and run: bootrec /fixboot and bcdboot C:\Windows. After reinstalling the bootloader, detach the ISO and confirm that the VM now boots directly from the virtual disk.
How do I prevent this issue in future installations?
For new VMs in Proxmox, ensure the boot order points to the virtual disk rather than the ISO before completing installation. When using UEFI mode, select “Use UEFI (OVMF)” and add an EFI disk during VM creation. This ensures that the bootloader is written persistently to the EFI partition.
I hope you found this guide very useful on how to fix “Unknown Error occurred when installing Veeam Software Appliance”. Please, feel free to leave a comment below.
