nearly every Sharepoint installation I have seen, the health analyzer shows the message “ Drives are at risk of running out of free space“, shown as Warning, or „Drives are running out of free space“, shown as Error.
Reason is, that there are two rules set up within SharePoint 2010 to check the free space. To get the Warning, the free space on one of the drives has to be lower than 5 times of installed RAM.
The Error-Message will come up, if there is less than 2 times of free installed RAM.
Installed RAM: 16GB
Minimum required free space without Warning: 5*16=80GB
Real free space: 79GB or less than that, you will get the warning.
Minimum required free space without Error: 2*16=32 GB
Real free space: 31 or less than that to get the Error.
The link to the rules you will find in another post of mine here.
An overview to the rules are given by Microsoft here.