Issue expanding raid6 array using storcli

I had the same problem as OP that I also struggled to diagnose. After performing my first successful expansion I attempted to add additional disks with the following command:

./storcli64 /c0/v0 start migrate type=raid6 option=add drives=0:6-7

Which responds:

------------------------------------------------------
VD Operation Status ErrCd ErrMsg
------------------------------------------------------
 0 Migrate   Failed   255 Start Migrate not possible.
------------------------------------------------------

I couldn't find an easy way to diagnose the actual fault. The only clue as to what might be going on is the following command:

./storcli64 /c0/v0 show all

You should get an output similar too:

VD0 Properties :
==============
Strip Size = 256 KB
Number of Blocks = 125002842112
VD has Emulated PD = Yes
Span Depth = 1
Number of Drives Per Span = 6
Write Cache(initial setting) = WriteBack
Disk Cache Policy = Disk's Default
Encryption = None
Data Protection = Disabled
Active Operations = Background Initialization (89%)
Exposed to OS = Yes
OS Drive Name = /dev/sda
Creation Date = 13-07-2021
Creation Time = 05:45:51 AM
Emulation type = default
Cachebypass size = Cachebypass-64k
Cachebypass Mode = Cachebypass Intelligent
Is LD Ready for OS Requests = Yes
SCSI NAA Id = 600304802411a602287fe50f14e63699
Unmap Enabled = N/A

In my case you can see:

Active Operations = Background Initialization (89%)

To get more details on this, execute:

./storcli64 /c0/v0 show bgi

This should give a similar output to the migration with:

-------------------------------------------------------
VD Operation Progress% Status      Estimated Time Left
-------------------------------------------------------
 0 BGI              89 In progress 17 Hours 53 Minutes
-------------------------------------------------------

It seems that once the migration completes, this does not mean the controller is ready, it must then run a background initialisation step.