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AC is different-- all users share a common database, which is forced onto the C drive so far as I can tell, and might not put anything at all into the SBS "folders" on the D drive, so it is probably bigger than it needs to be, at least for our implementation.

Nope, it's not lol. It's just that everyone hits next on the install wizards ha. If you look closely it asks you for the location.

For SBS-E, try to only keep the OS on C:, avoid installing any extra programs because as you said that partition is only 60GB. Resizing the partition is difficult and it's not something you can do in Windows. If you wish to expand the drive, check out this video: http://www.youtube.com/watch?v=q1GP1qOReKA

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SBS Essentials arbitrarily partitions the main drive (and doesn't invite changing partition sizes, though perhaps it could be forced.)

The C partition is 60GB. Whatever is left goes to the D drive. 160GB minimum. They wanted a one click setup for users who aren't familiar with hard drive sizes. The only way to change it would be before the install with an answer file. The problem with that is you need to be familiar with re-authoring a disc image which, in turn, was another reason I recommend partitioning the drive the way I did.