Hi. Apologies for delay in responding.
Yes I did check for that and there was none. It occurred on at least 2 VMS I tried. In one case just shutting a VM down and trying to restart gave the missing vdi error. Everything I checked (with my limited knowledge of the CLI) looked fine. As soon as I rolled back to 7.4 on the master the VMS started without any issue.
Its as if the upgrade applied some schema changes to the vdi linking in some way perhaps?
I did try a second time just updating the master but with the same result - stopping and trying to restart a VM on any host (we have 4) gave the vdi missing error. Again rollling back fixed the problem but leaves me worried about upgrading.