To convert your E drive from MBR (Master Boot Record) to GPT (GUID Partition Table) without losing any data, especially since you've tried using 
mbr2gpt and encountered an issue, let's walk through alternative methods and address potential problems.
### Method 1: Using Windows Command Prompt (Diskpart)
Using 
diskpart is straightforward but can be risky if the tool mandates deleting partitions. Since you want to avoid data loss, I recommend using this method only if you're absolutely confident about the drive's state and data backup.
1. 
Open Command Prompt: 
   Press 
Win + X and select "Command Prompt (Admin)" or "Windows PowerShell (Admin)."
2. 
Run Diskpart: 
   Type 
diskpart and press 
Enter.
3. 
List Disks: 
   Type 
list disk and press 
Enter. 
   Identify the disk number that corresponds to your E drive.
4. 
Select the Disk: 
   Type 
select disk X (replace X with the disk number) and press 
Enter.
5. 
Convert to GPT: 
   Type 
convert gpt and press 
Enter. 
   If you see a message indicating existing partitions must be deleted, 
do NOT proceed with this method to avoid potential data loss.
### Method 2: Using AOMEI Partition Assistant
AOMEI Partition Assistant is a third-party tool that does a splendid job of converting MBR to GPT without losing data, and it is very user-friendly.
1. 
Download and Install: 
   Download AOMEI Partition Assistant from 
the official website and install it.
2. 
Launch the Software: 
   Open AOMEI Partition Assistant.
3. 
Find Your Drive: 
   Locate your E drive from the list of drives.
4. 
Convert to GPT: 
   Right-click on the E drive and select "Convert to GPT."
5. 
Apply Changes: 
   Click on the "Apply" button to perform the conversion.
### Method 3: Addressing MBR2GPT Validation Error
Your attempt with 
mbr2gpt failed validation due to a layout issue. Here are steps to troubleshoot:
1. 
Check Disk Health: 
   Run 
chkdsk E: /f to check if there are any file system errors on the E drive. Replace 
E: with your drive letter.
2. 
Free Up Space: 
   Ensure there’s enough unallocated space on the drive. Sometimes, having a small partition at the end can cause layout validation to fail.
3. 
Third-Party Software: 
   If you're still facing issues, or if 
mbr2gpt continues failing validation, using a third-party tool like AOMEI Partition Assistant might be a more straightforward option.
### Important Notes:
- 
Backup Your Data: Always ensure that you back up your data before making changes.
 
- 
Post-Conversion Check: After conversion, check your E drive by navigating its files.
- 
Non-Boot Drive: Confirm again that your E drive is not the boot or system drive.
By following these methods, you should be able to convert your non-boot E drive to GPT without any data loss. If any part seems unclear or you're facing other specific issues, just let me know!