I will backup again without deleting the snapshot and then attempt a differential restore.
I will update this thread when done.

Thank you,
I will consider that as a way forward.

I started with XCP 1.0 a few years ago then 1.1 and 1.6 'till xenserver 6.2 became open source. Been with xenserver ever since. I run a small (one man band) company providing IT and sysadmin support to fourteen small to medium sized companies. All are virtualized on various versions of xenserver. All are using standalone servers. Some more recent ones use bcache local storage with redundant mdadm array as backing device and ssd as caching device. /dev/bcache0 then becomes local repository so all vms in repo benefit from ssd caching. Citrix pissed me off with the release of xs7.3 so I am really pleased to find this fork. Having started with XCP 1.0 it feels like coming home.
I came into IT nearly 20 years ago having spent the previous 30 years in electronic engineering. I am self taught. I have to say I found electronics easier...

@dvdhngs
Run lvscan to see if there are any active logical volumes. If there are run vgchange <group name> -an to deactivate all logical volumes. Now try to stop the array.

@frank-s Hello again. My issue is resolved. I was using used disks which had previously been used with mdadm. Although I applied "mdadm --zero-superblock" to every partition and the device itself before deleting the partition tables and dd zeroed the first 50 Gigs of the drives, the installer didn't like it. Maybe I should also have zeroed the end of the drive??? Anyway I zeroed out the entire drive (both of them) with dd and the installer worked like a charm. I also tested upgrade which also worked flawlessly.
Thank you for the great work you are doing. Over the years I have spent many many hours loading xenserver with bootable raid. This is going to make life so much easier...

@Danp Thanks for pointing me in the right direction. I was following an older howto where --global was not specified. That was my first problem - now fixed!
Second problem was that I had installed into the root directory of Centos7 so the paths for creating the links were different. Once I had found the correct paths and created the links all was good.
Thanks again.
Frank.

@olivierlambert <The snapshot is deleted. You need a reference point to do differential>. So, since the snapshot is deleted, create a new one (snapshot) as part of the restore process and use that for the reference - then delete it when done.

@olivierlambert Agreed, but in the case where the snapshot has been deleted, why can't the differential restore process simply create a snapshot from the vm and use that instead?

@olivierlambert So, earlier in this thread, florent said "differential restore work by not transferring the data that the snapshot and the backup have in common, without reading any of those
it clone the last snapshot, an then revert the block changed between the older backup and the snapshot". If this is true (and I'm sure it is) why can't the differential restore system create the missing snapshot and use that to complete the differential restore?

@olivierlambert No, not recreating the snapshot from the CBT metadata but simply creating a new temporary snapshot, perform differential restore and delete the temporary snapshot when done.

@frank-s So- I have been thinking about this. If an NBT/CBT backup is set to destroy the snapshot, would it be possible to make differential restore work by having the restore process create the snapshot it needs, perform the differential restore and then delete the snapshot?

Thank you.

Thank you. Is there somewhere I can read about this instant restore feature?

@frank-s Soz - I meant differential restores...

@florent
I just did a delta backup on a small VM but chose NOT to delete the snapshot. It completed quickly and I then tried a differential restore and it worked. I also tried to do a differential restore on a delta backup made yesterday when the snapshot was deleted - this also worked. Will it be possible in the future to make differential backups work without keeping the snapshot? Is such a thing possible?
Thanks,
Frank.

I will backup again without deleting the snapshot and then attempt a differential restore.
I will update this thread when done.