Recently I have a Dell machine XPS8900 upgraded to Windows 10 Anniversary edition. But it was not all smooth without some issues, of course.
In May 2016, I cloned its original Windows 10 disk onto an SSD drive. I had the system booted up the first time from the SSD and it worked flawlessly for a couple of months.
The SSD is on SATA 0. The original Windows 10 HDD remains as a 2nd hard disk, connected to SATA 1. I have another 2 disks installed in the system and together, these 3 HDDs are configured into a RAID 5 array. No formatting of the original Windows 10 HDD as I want to preserve all its content.
Then when Windows 10 Anniversary update came along for an update, the update failed.
After some hours of troubleshooting, the root cause was that I had 2 EFI System Partitions on the system, one on the SSd, and the other was on the original Windows 10 HDD. This confused Windows 10 update but it wouldn't tell you straight that it didn't like it.
And so, Windows update failed and gave an error stating some SAFE_BOOT stuff. Unfortunately I didn't capture a screenshot, otherwise it will be most useful.
Now, the solution is simple: change the other partition ID.
EFI System Partition has this unique GUID as its identifier.
I ran a program called diskpart.exe from the command prompt, as an administrator. While in the program, SELECT the disk that represents my 2nd HDD (an RAID 5 array). LIST its PARTITION. SELECT the EFI partition by referencing its number, and issue a command "uniqueID disk" to show its current identifier.
The identifier ID shown matched the above.
The same command can also be used to change its UNIQUE ID:
uniqueID disk ID=c12a7328-f81f-11d2-ba4b-00a0c93ec930
The only difference is the last character. I changed it to a 0.
Exit DiskPart and retried the Windows 10 Update. It worked.