In un thread della community americana ho trovato questo suggerimento per il mio problema. Ora effettuo un tentativo cercando di documentare ogni passaggio.
UPDATE:
I tried the conversion from within Windows and everything went smoothly, including creating the EFI partition, but received the following message at completion: "MBR2GPT: Failed to update ReAgent.xml, please try to manually disable and enable WinRE."
Checked WinRE status and it was disabled. So, entered the command to enable WinRE and received the following message: "REAGENTC.EXE: Unable to update Boot Configuration Data."
However, everything seemed to be working just fine, including the Recovery Environment. Rebooted and it went into Repair mode for a few seconds then loaded the OS. Checked the WinRE status again and it is Enabled. Everything now appears to be fine and the system drive has been successfully converted from MBR to GPT.
Thank you Sumit for all your advice, and thank you Saltgrass too for responding. I have included this last post for others who may face a similar issue. It seems to me that the common advice of running MBR2GPT outside Windows is NOT the best method as even the most minor of issues will cause it to fail and you end up with an unbootable system. Whereas running MBR2GPT from within Windows seems to work a lot better. At least, it has for me, on two occasions.
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