just for the records, busy lun have nothing to do with VSS retryable error.
VSS writer is Exchange, nothing to do with the underlying storage.
when you have a Microsoft Exchange VSS Writer in the following state:
Writer name: 'Microsoft Exchange Writer'
Writer Id: {76fe1ac4-15f7-4bcd-987e-8e1acb462fb7}
Writer Instance Id: {cc2dd7f9-68d3-4c6a-87b9-c32b81b25e2e}
State: [7] Failed
Last error: Retryable error
it means Exchange writer consistently disallows backups and you need to clear this state before trying the next backup.
typically Exchange Information Store Service needs to be restarted. Possible also other Microsoft Exchange related services need a restart.
So, it makes sense that your exchange server reboot cleared this VSS writer state indeed.
Domenico.