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

Windows Disk Management often refuses to remove EFI partitions. A bootable Linux environment with GParted is another option: identify the correct 4 TB SSD, delete only the old EFI partition, apply the changes, and then resize or extend the desired partition. Do not delete or format the partition containing your existing files. If the EFI partition is the one Windows currently uses, leave it in place and only remove the Linux boot files instead.

The error can also happen when the wrong object is selected or when the partition is not actually the one you think it is. Run DiskPart from an administrator terminal and inspect the output of `list disk`, `select disk X`, `detail disk`, and `list partition` before changing anything. If the partition is on a dynamic, Storage Spaces, or otherwise unusual disk, use the appropriate storage-management tool rather than forcing a deletion.

Be careful before deleting it: an EFI System Partition can contain the Windows boot files as well as the old Linux bootloader. If Windows and Arch shared the same EFI partition, deleting it could make Windows unbootable. First check which disk and partition you selected with `list disk`, `list partition`, and `list volume`, and make sure you have a backup. If it is definitely an unused EFI partition, DiskPart usually removes it with: `select disk X`, `select partition Y`, then `delete partition override`. The disk and partition numbers must be verified carefully. Afterward, you can extend an adjacent data partition into the unallocated space if the layout allows it.