Let's keep everything on this Windows error in the same topic instead of creating multiple.
virtualwatts wrote:Is the HotAdd scsi controller limit still valid in v5.0.1?
It's not a Veeam limit, but VMware iSCSI controller's limit, so it does not depend on Veeam version.
virtualwatts wrote:How does Veeam decide which scsi controller to use for the Hotadd when there are multiple (assuming it ever leaves 0:xx)?
Veeam doesn't decide this, it happens under the hood in the vStorage API for Data Protection (or, possibly, in vSphere).
virtualwatts wrote:Why the Windows No Disk error message on a Hotadd failover to NBD? What is the specific cause of that?
I don't really understand what you are asking here. But the cause of this Windows error is currently unknown (aside of what is already written above in this topic). And NBD failover is caused by the fact that primary transpost mode (Hot Add) fails because of this error.
In your case I would suggest removing paravirtual iSCSI controller from Veeam Backup VM, as I've seen reports of this one causing various issues before.