Hi @OzDownUnder ,
Thank you for reaching out to the Microsoft Q&A platform. Happy to answer your question.
From the error details, it appears there has been a major change made for the "osdisk_1" on the VM while the replication was in progress. Due to which the replication is unable to resync. In a situation like this, the ideal thing to do is disable the replication and re-enable the replication from scratch to make sure the DR disk are up to date.
Note: It is recommended that there should be no changes done on the disks being replicated by ASR while replication is in progress.
If it is azure VM, follow the disable replication procedure from here and Enable protection
If it is on-premises VM, then follow the procedure from this link and Enable protection
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