5 VMs are failing out of 12 with Failed to process, Error: Cannot find full backup.

From your description, the issue seems to be on the Backup Copy (target) side rather than the source.

Even if the source job has valid .vbk files, the Backup Copy Job maintains its own chain on the target repository. If the full backup for those VMs is missing, was removed by retention, or the mapping was lost, you will see this error.

I would recommend checking if there is any existing chain for those VMs on the target repository (even orphaned).

If no valid chain exists, the cleanest approach is to trigger a new Active Full to rebuild it. In some cases, if mapping is not possible, removing and re-adding the VMs may be required.

There is no .vbk file present in the target repo for these 5 VMs, concern with triggering active full for the copy job is that we have limited space in the target USB drive and also this copy job is tied to 2 source backup jobs of which the issue is only with 5 VMs of one job. Kindly advise.

If there’s no vbk file present for those 5 VMs in the target repository, the backup copy chain is effectively broken for them, so an Active Full would indeed be required to re-seed the chain.

However, since you’re constrained by space and the issue only affects a subset of VMs from one source job, one option could be to temporarily create a separate backup copy job for just those 5 VMs and run an Active Full only for them. This helps avoid impacting the rest of the chain and minimizes space usage.

Alternatively, you could also review retention and free up some space, or plan the Active Full more strategically — for example, aligning it with a maintenance window or existing full backup cycle (if applicable in your environment).

upon further checking , seems the .vbk files for the 5 VMs are only not appearing in the backup copy job -- disk ( backup copy ), when checked further there are indeed .vbk and .vbm files for these 5 VMs in the USB repo , the files are not visible in the disk ( backup copy ).

That’s an important detail.

If the vbk and vib iles are present in the repository but not visible under the backup copy job, this might indicate a metadata or chain recognition issue rather than missing backups.

Before considering an Active Full, I would suggest trying a rescan of the repository.

Also, since this is a USB repository, it might be worth checking if there were any changes to the drive letter or mount path, as that could cause the job to lose track of the backup chain.

From a best practices perspective, USB repositories can sometimes introduce challenges like this due to their nature, so if possible, it might be worth considering a more stable repository type for long-term reliability.