Cannot copy Photos library from Drobo 5N

I am trying to take a copy of my Photos Library Sparsebundle ahead of Mac OS Catalina

The Photo Library is on a Drobo 5N and works fine with Photos itself, and has just added the latest photos to it with no problem.

However when I try and copy from the Network Share to an external AppleFS formatted drive the copy fails.

The Library is 168GB in size and the USB drive has a 256GB capacity and is empty,

The copy gets to 164GB overnight then gives the error:

"The operation can't be completed because you don't have permission to access "Photos Library.sparsebundle".

At this point the USB drive capacity suggests that 164GB has been copied. However, on clicking OK in the dialog the file disappears after a couple of minutes and the drive capacity returns to full.

Any ideas? As far as I can see I can't change the permissions on the file on the Drobo because it's formatted in DroboFS. My user has read/write access to the USB drive