On a Small Business Server 2011 machine (that’s Server 2008 R2 base), I’m getting warning event 51 from Microsoft-Windows-Backup: “The backup storage location is running low on free space. Future backup operations that store backups on this location may fail because of not enough space.”
The 2TB destination disk has 15% free space. That’s after deducting 15% in shadow copy storage space used by Windows Server Backup.
Researching this a bit, I found this 2010 thread in which a Microsoft employee states:
If the current backup directory occupies 60% of the backup target space (local HDD), this alert is given. It is given, as the recommended target size should be 1.5 times the backup source size, to maintain a reasonable number of backup versions and lowering the chances that the backup fails because of the disk getting full. Changing the warning threshold is not possible.
The backup source is about 1200GB so the available target of 1811GB is 1.5 times the size. I don’t understand where they came up with the 60% space rule. Imagine if it was an 8TB target drive. I’d need to keep 3.2TB free to avoid this warning?
I checked a Server 2016 machine that has “only” 36% free on the target disk and the warning does not appear. So apparently this behavior has been changed since 2010. For now, I’ll just have to ignore this alert on the older server and monitor disk space separately.