Access denied on EFI system partition

On a Windows 8 system I tried to access an NTFS partition that had been copied into the disk. Disk management denied adding drive letter and when I assigned one using diskpart all I got was access denied when trying to access the root of the drive. Windows recognized the drive as EFI system partition - totally wrong.

Turned out the problem was that the partition was marked with the boot flag. I removed it using gparted in Mint/Ubuntu and then I could access the drive.