You can expand RAID with more than one additional drive simultaneously

I moved a 2-drive RAID1 volume from a 2-bay chassis into a 6-bay one and wanted to expand the volume to 6 drives.  Normally, that would mean 4 re-syncs.  But I've been looking at some of the Netgear-specific command-line utilities and found that volume_util has some interesting options.  I decided to give expanding to more than one additional drive simultaneously a try, since volume_util --help seemed to indicate it could be done.  And it sure can, at least if you have a single-layer RAID (I think you could with more, but I haven't tried it).  And it's really simple.

Now for this to work, you have to be in FlexRAID mode (XRAID off in Volumes page).  If you don't do that, then the NAS will start to expand the volume as soon as you insert one drive, before you can tell it to do something else.  NOTE:  If you have a volume that has been vertically expanded, you cannot go back to XRAID after turning it off, so keep that in mind before you proceed.

With my existing volume data on the drives in bays 1 and 2 (which are 0 and 1 from the command line), I inserted 4 more same-size drives and formatted them via the GUI.  Then, from SSH, I issued this command:

volume_util -E data 2,3,4,5.  My NAS is now syncing all 6 drives into a RAID5.  The GUI and SSH (lsblk and cat /proc/mdstat) show that.  I was also already able to switch back to XRAID.

If it turns out I'm premature and all isn't fine when the sync is done a couple days from now, I'll post a follow-up.  But I have confidence that I will have expanded from 2 to 6 drives in a single re-sync in about the same amount of time it would have taken to expand to just one more under XRAID and far less time and drive stress than 4 separate re-syncs.