Hello Johannes Gorissen,
Greetings! Welcome to Microsoft Q&A Platform.
I understand that you are not able to find the managed storage account created while VM creation for storing Boot diagnostics.
When you enable boot diagnostics for a VM and configure it using 'enable with managed storage account (recommended)', a managed storage account is created automatically by Azure. This storage account is not visible from the portal and is created in a system-generated resource group. The storage account created by Azure for boot diagnostics is not publicly accessible by default**.** This storage account will not be directly accessible to the user and will be the default option when creating a VM via the portal going forward. The only limitation present should be bi-directional serial console access (which if needed, will require the user to switch to their own storage account).
The system-generated resource group that contains the storage account for boot diagnostics is created under the subscription scope. When you create a VM in Azure portal, boot diagnostics is enabled by default. The recommended boot diagnostics experience is to use a managed storage account, as it yields significant performance improvements in the time to create an Azure VM. An Azure managed storage account is used, removing the time it takes to create a user storage account to store the boot diagnostics data.
The Boot diagnostics feature does not support premium storage account or Zone Redundant Storage Account Types. If you use the premium storage account for Boot diagnostics, you might receive the StorageAccountTypeNotSupported error when you start the VM.
An alternative boot diagnostic experience is to use a custom storage account. A user can either create a new storage account or use an existing one. When the storage firewall is enabled on the custom storage account (Enabled from all networks option isn't selected). refer article.
Boot Diagnostic Storage only contains serial log information and screenshots. a .bmp (a screenshot) and a plain text file (text/plain) of the serial console log. The VMs Disk content isn't stored via boot diagnostics.
When you disable Boot Diagnostics, it stops capturing screenshots and logs from the VM during boot and shutdown. The container and blobs get deleted. The storage account is completely disassociated with the VM and/or deleted.
Because this is a "managed" storage account, it is managed by Microsoft and not visible to customers. The only way to control the access to the boot diagnostics data is, through this RBAC action: Microsoft.Compute/virtualMachines/retrieveBootDiagnosticsData/action It retrieves boot diagnostic logs blob URIs.
Virtual Machines - Retrieve Boot Diagnostics Data
Also, note that users can't configure a retention period for Managed Boot Diagnostics. The logs are overwritten when the total size crosses 1 GB.
To see the log information of custom storage account,
Go to the virtual machine blade in the Azure portal, the boot diagnostics option is under the Help section in the Azure portal. Selecting boot diagnostics display a screenshot and serial log information. The serial log contains kernel messaging and the screenshot is a snapshot of your VMs current state. Based on if the VM is running Windows or Linux determines what the expected screenshot would look like. refer article
Please consider checking the below steps to troubleshoot the issue if not able to see the logs,
Verify the logged in account does have permission to access storage or update settings in virtual machine.
Check if you have enabled ADLS Gen 2 in your Storage account, if so (not currently supported as a destination for diagnostic settings even though they may be listed as a valid option in the Azure portal). This article will help you in Diagnostics data is not logged to Azure Storage(Determine if none of the data is appearing or some of the data is appearing.)
Additional information: Log data is stored in either Blob or Table storage. For more information on Store and view diagnostic data in Azure Storage see here.
Note: If diagnostic data isn't being collected or you're having trouble viewing it in the portal, reinstalling the agent might help. This removes the agent but keeps all existing diagnostic data in your storage account. After the agent is removed, you can re-enable diagnostics for this virtual machine.
Similar threads for reference - https://stackoverflow.com/questions/40705537/azure-vm-stopped-unexpectedly-says-storage-account-xxxxxxxx-not-found#:~:text=This%20issue%20may%20be%20because%20the%20diagnostics%20storage,Support%20%2B%20Troubleshooting%20tab%20in%20the%20VM%20blade., https://stackoverflow.com/questions/66467647/can-t-find-storage-account-details-for-my-azure-vms, https://stackoverflow.com/questions/49362468/change-the-boot-diagnostic-storage-account-of-azure-virtual-machine.
Hope this answer helps! Please let us know if you have any further queries. I’m happy to assist you further.
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