After performing the “diskpart extend” command to merge two partitions the new partition will display in disk management however will not show the full capacity. This is a known problem if the command was run without sufficient system resources.
The partition size is extended, but the file system remains the original size when you extend an NTFS volume” – Unfortunately Microsoft have pulled the original “KB832316” (As of 2021) so there is limited information available. There are some references for diskpart here: KB325590

The following method of fixing this with the diskpart tool may work for some (but not others)

diskpart
list volume
select volume X
extend filesystem

If like me you received the following error “Diskpart failed to extend the volume. Please make sure the volume is valid for extending” then there is a 99% this method will not work, in this case you can use the FSEXTEND.EXE tool, after burning around the net and looking at the following EE article it seems that getting hold of the tool is another problem. I resolved this by 45minutes of talking to Microsoft and getting a case open… but to avoid this I’ve uploaded the tool…

Trying to get FSEXTEND.EXE ?

• Get it here: FSEXTEND

How to Use:

The FSExtend tool really is a “one trick pony” if you try to get the switches required by the program it will simply return with “usage: fsextend.exe driveLetter” so you just need to perform the following:

DISKPART> select volume 1
DISKPART> extend filesystem
DiskPart successfully extended the file system on the volume.
DISKPART> exit
Leaving DiskPart...