This will happen if the EFI partition has the wrong GPT partition type. As it turned out, this was also causing the problem with the cumulative update.
If you are having similar issues, you can check whether this is the problem with the
diskpart command. Assuming you have only one hard disk drive,
select disk 0
You should see something like this:
Partition ### Type Size Offset
------------- ---------------- ------- -------
Partition 1 System 500 MB 1024 KB
Partition 2 Primary 445 GB 501 MB
The smaller partition named "System" is the one you want, usually partition 1, so
select partition 1
And you should see something like this
Type : c12a7328-f81f-11d2-ba4b-00a0c93ec93b
Hidden : Yes
Attrib : 0XC000000000000000
Offset in Bytes: 1048576
Volume ### Ltr Label Fs Type Size Status Info
---------- --- ----------- ----- ---------- ------- --------- --------
* Volume 3 FAT32 Partition 500 MB Healthy System
If the file system is not FAT32 then you are not looking at the right partition. It should also be Hidden, and will not usually have a drive letter assigned unless (as in the originally posted question) it has been explicitly given one for troubleshooting purposes. It might not be exactly 500MB, but should only be taking up a small fraction of the hard disk.
The type of the EFI partition should be
c12a7328-f81f-11d2-ba4b-00a0c93ec93b as shown above. If it is not, and in particular if it is
ebd0a0a2-b9e5-4433-87c0-68b6b72699c7 (see Microsoft Basic Data Partition on Wikipedia) then that is likely to be the cause of the problem.
If the partition type is incorrect, you can fix this with the
set id command,
reagentc /enable command should then work, and if you are lucky, so will Windows Update.