On many of my early installations of SBS2008, I had the unfortunate situation of not allocating enough hard drive space on the C: drive. My opinion has always been to move data and other system information out of the C: drive to other drives, and keeping the C: drive to a reasonably small size. I though that size was 60GB. I was wrong. I would recommend setting the C: drive to at least 80GB or more.
However, if you do have a 60GB partition on the C: drive, one of the first signs of trouble is when the free space on C: drops below 2GB. Exchange 2007 stops receiving external emails. This is caused by a condition known as Back Pressure. This is described in more detail here – http://technet.microsoft.com/en-us/library/bb201658(EXCHG.80).aspx
The easiest way to remedy this situation is to immediately free up disk space. Fortunately, there are some great articles on how to do this.
- Reclaiming Disk Space Lost to IIS Logs on SBS 2003 and SBS 2008 – http://blogs.technet.com/sbs/archive/2008/02/28/reclaiming-disk-space-lost-to-iis-logs-on-sbs-2003.aspx
- How to Move WSUS Content and Database Files to a Different Volume – http://blogs.technet.com/sbs/archive/2009/09/23/how-to-move-wsus-content-and-database-files-to-a-different-partition.aspx
Other things, which should have already been done via the SBS Console is moving the Exchange database, Windows Update Repository and Sharepoint database out of the C: drive. They can be found here.