Software⏱️ 3 min read📅 2026-06-19

How to Fix: Attempting to passthru USB disk to XENserver windows guest, reporting wrong size and error

Passthru USB disk issue in XENserver Windows guest, wrong size and error reporting.

Quick Answer: Check if the disk is properly attached to the virtual guest and ensure the correct partition type is used.

📋 Table of Contents

  1. ⚠️ Common Causes
  2. 🚀 How to Resolve This Issue
  3. 💡 Conclusion

The error described occurs when attempting to passthrough a USB disk to a Windows guest in XEN 6.2, resulting in incorrect size reporting and I/O device errors. This issue affects users who are trying to utilize large storage devices with their virtual machines.

This problem can be frustrating for administrators as it prevents them from utilizing the full potential of their storage devices within their virtual environment. To address this issue, we will explore the root causes and provide a primary fix method.

⚠️ Common Causes

  • The incorrect size reporting is likely due to the fact that the Windows guest is not able to read the GPT partition table correctly. This can be caused by the disk being listed as 'Use GPT due to size' in fdisk -l, which may indicate that the guest OS is not properly configured to recognize the GPT partition type.
  • Another possible cause of this issue could be related to the way the disk is attached to the virtual machine. The fact that the disk appears in Computer Management as an uninitialized disk with a large size suggests that there might be an issue with how the disk is being recognized or mapped by the guest OS.

🚀 How to Resolve This Issue

Correcting GPT Partition Type and Re-Attaching Disk

  1. Step 1: Using fdisk -l again, verify that the disk is still listed as 'Use GPT due to size'. If it has changed, re-run gdisk to ensure that the correct GPT partition type is set.
  2. Step 2: In XenCenter, detach the disk from the virtual machine and delete any existing LUNs associated with it. Then, re-attach the disk using XenCenter's 'Attach' functionality, ensuring that the correct LUN ID is selected.
  3. Step 3: After re-attaching the disk, verify that the disk appears in Computer Management as a properly initialized disk with the correct size.

Alternative Fix Method: Using a Different Disk Type

  1. Step 1: If the above method does not work, try using a different disk type, such as MBR (Master Boot Record), to see if it resolves the issue.
  2. Step 2: Using gdisk or fdisk, create a new partition table on the disk with an MBR type. Then, re-attach the disk to the virtual machine using XenCenter's 'Attach' functionality.

💡 Conclusion

To resolve the error of incorrect size reporting and I/O device errors when passthrough-ing a USB disk to a Windows guest in XEN 6.2, try correcting the GPT partition type and re-attaching the disk or using an alternative disk type. If these steps do not work, further troubleshooting may be necessary.

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