A Free Disk Space alert is a is a Threshold Alert that notifies you by email when your deployment has used more that some percentage of the available disk space for more than some number of minutes. This creates an Incident in the Managed Solr Dashboard.
When a Solr deployment runs out of disk space, it may simply crash. Also, much lower levels of disk usage (below half) can make backups impossible.
Large Index Size
Some indexes are static, but most grow over time. Eventually, a growing index will strain the disk space allocated to it. When that happens, we can upgrade the deployment or just add disk space.
Incidentally, replicas of the same index do not necessarily occupy the same amount of disk. See Nodes Use Different Disk Space for more depth on that subject.
The Backup process is extremely disk-intensive. It can easily cause a Free Disk Space alert.
Add Disk Space
It is possible to add disk space to the nodes of a deployment by making a request of SearchStax Support. We can add disk space in blocks of 50GB for $25/node/month.