I like to share a small trick; I use in my Exchange Servers or no matter any servers where we have some critical application running. I keep a dummy folder with 5 GB data in my Exchange Database and log drives. This folder contains 5 GB of any kind of data which I can delete whenever needed.
Then I have the SCOM monitoring setup for alerting me when disk space goes below 5 GB. So in reality I get an alert when the disk free space go below 10 GB and get enough time to plan the disk expansion. (Probably you might think why I am not setting up an alert for 10 GB free space in this case, its just because I have my other admins while I am away and they might feel little relaxes with their planning when they see 10 GB free space. also if we doesnt keep the dummy data its free space available for application and exchange transactions or an mail box import can directly fill up the free space and get me trouble)
Being said this let me also mention, now a days with Server and SAN virtualization technologies expanding a disk is not a major issue. But we are humans and in case you missed an alert and in a kind of situation your san admin is not around this buffer space will help especially if you are an admin like me looking after multiple services. So it may be soon a old school trick which we veterans talk about 🙂