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Azure MARS: must drive replacement force ugly resync?

Frank Iannarilli 20 Reputation points
2025-10-15T01:33:34.51+00:00

Have successfully for years been using MARS from home PC for Azure Backup.

Just replaced a drive (hard to SSD) but kept same drive letter (E:) and copied over same folder hierarchy from old to new drive. But now MARS backup doesn't work for those files/folders on E: slated for backup. (I backup both C: and E:, and a backup-now job completes with warning - the C: succeeds, but the E: fails).

To attempt solution, I first upgraded my MARS Agent to 2.0.9415.0 per https://learn.microsoft.com/en-us/azure/backup/upgrade-mars-agent (I had weeks ago upgraded-in-place from Win10/Pro/64 to Win11, in case that's pertinent).

I retried changing the files/folders in MARS>ScheduleBackup>MakeChangesToBackupItemsOrTimes> to no avail - it kept tripping over non-existent "old" file paths burned into its config memory and would not accept the revised definition.

Upon searching for pertinent terms/phrases, Google AI Overview indicated that MARS ties the files/folders specified for backup to the drive GUID, and contrarily that it ties only to the drive letter (which is better for an end-user like me). Evidently, the former is true (arggh!), which led to trying to force a "resync" via Registry Editor per here: https://learn.microsoft.com/en-us/answers/questions/842493/initial-backup-failing-on-three-on-prem-vms (Reviewing the MARS log file does actually show/correlate the drive GUID with the ReplicaID as suggested). This didn't work either (even after quitting MARS and stopping/restarting the two MARS services in services.msc).

There was also Google AI Overview slop (?) indicating that right-clicking on a failed job in MARS Agent would yield a resync button - that would actually be wonderful, but it doesn't exist.

I ultimately had to do MARS>ScheduleBackup>StopUsing...DeleteStoredBackups, then start from scratch with defining a new scheduled backup.

It would be nice if backup jobs were internally defined based on drive letter rather than GUID. And if it need be GUID, then to have the advertised 'resync' option. Or to make this into a question for the forum, is there a way to have avoided this mess upon drive replacement?

Azure Backup
Azure Backup
An Azure backup service that provides built-in management at scale.
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  1. Vinodh247 40,031 Reputation points MVP Volunteer Moderator
    2025-11-28T15:24:23.9033333+00:00

    Hi ,

    Thanks for reaching out to Microsoft Q&A.

    Yes. If you replace a drive, MARS sees it as a completely different volume because it tracks backup items by volume GUID, not drive letter. That is why your E: paths refused to validate even though the letter was the same. The MARS catalog still pointed to the old GUID, so backups for that volume could never succeed.

    There is no supported way to make MARS rebind a backup set to a new volume GUID. Clearing registry keys or restarting services will not fix it. The “resync” references floating around online apply only to consistency checks on existing volumes, not volume identity changes.

    The only supported fix is exactly what you ended up doing:

    Stop using the vault for that machine.

    Delete stored backups.

    Reconfigure the MARS backup from scratch, letting it detect the new volume GUID.

    If you want to avoid this in the future, the only real option is to avoid drive-level replacement scenarios where the GUID changes. MARS does not support “reattaching” a backup definition to a new disk. Drive letter continuity does not matter; GUID continuity does. Unfortunately, there is no hidden “resync volume mapping” feature today.

    Please 'Upvote'(Thumbs-up) and 'Accept' as answer if the reply was helpful. This will be benefitting other community members who face the same issue.

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