What happens as a database grows to the HDD's lmit?
When the hdd fills the DB could no longer write and it will crash. 
But that is the easy thing to solve &amp; monitor- whenever you reach 85% capacity just add more or clear old data. 

The harder/more important part is to ensure performance is not impacted by the growth in data (more rows to scan)
Usually this is handled by partitions and cleanup processes which somewhat mitigate the first issue as well. 

Regards 
Jony