We had failed data base copies on a CCR cluster and running Update-StorageGroupCopy failed. The backups have not run for a while but I could verify that the logs were indeed present. The best way to get around this is to run an Exchange backup and the Update the Storage Group Copy. This was not possible as wee needed to urgently get the database copies healthy. We then had to dismount all the databases, checked that they are in a clean shut down state using eseutil.exe /mh and move the logs to a different folder. We then mounted the databases and were then able to seed the database copies.