Time Machine on a local drive, then make that drive a network drive (via Server.app) on a different computer I have two macs running OS X 10.10.3. One has Server.app installed, is connected to the internet via ethernet, and also has a 2 TB disk attached via USB, with two partitions - "Backup" and "Storage". My other mac is a laptop used for schoolwork.

Using the Time Machine feature of Server.app, I was able to use the standard Time Machine interface to begin backing up my local mac onto the "Backup" volume on the server mac over the network.

However, I discovered that it takes 7 minutes to copy a 1 GB file over AFP, so backing up 300 GB on my local mac would take at least 35 hours.

So, I'm trying to figure out a way in which I could do the initial 300GB backup onto "Backup" over USB, and then connect that external hard drive to the server mac and continue doing backups over the network. That way, the incremental network backups would be much smaller than 300GB and could be done in a reasonable time period, e.g. overnight.

Any suggestions? I read the man page for tmutil but I'm not very comfortable with it just yet.
This MacSales article covers how to do this very well: 

http://blog.macsales.com/18406-speed-time-machine-past-88mph-over-your-local-network

Essentially, you start with a fresh Time Machine backup. Start it on Wifi, then stop it, as it begins to backup files. This basically establishes the Time Machine archive. Then you attach your hard drive (or laptop) to the wired network, and resume backup. Once you have the initial backup done over the much faster wired network, subsequent backups will be much faster, as the are much smaller.

You establish the initial Backup over wifi because Time Machine does things differently over wifi vs directly connected. 

In any case, best of luck to you: I have never managed to get Time Machine to work reliably over wifi, and especially if you tend allow your Mac to go to sleep during a Time Machine backup (shut the lid for example)