How can I safely remove a leftover EFI partition and reclaim the unallocated space?

First, make a backup of anything important and confirm the disk and partition numbers carefully. The EFI System Partition may still be needed by Windows, so do not delete it just because it contains an old Linux bootloader. In DiskPart, use `list disk`, `select disk N`, and `list partition` to inspect the layout. You can also check which EFI partition Windows is using with `mountvol S: /S`; if it contains the active Windows boot files, leave it alone. The unallocated 699 GB is not inaccessible—it can be added to an adjacent partition only if the free space is directly next to it. If another partition, including the EFI partition, is between them, use a reputable partition manager that can move partitions, or back up and recreate the layout. Avoid formatting or deleting any partition until the layout and backups are verified.

The error usually means DiskPart is refusing to remove a protected EFI System Partition, not that the command syntax is wrong. If you have verified that it is an abandoned Linux EFI partition and not the one Windows boots from, you may be able to remove it from a proper partition-management environment rather than from the running Windows installation. However, deleting it alone will not automatically merge the 699 GB into another volume; the target partition must be extended afterward, and it may need to be moved first. Take screenshots of Disk Management or `diskpart` output before changing anything, and do not use `clean`, `format`, or partition deletion on the disk containing your data.