If you put the maximum possible on a separate drive on a SQL install, you will end up with about 3GB on the system drive and about 800MB on your other drive.
After you have installed a couple of Service Packs and maybe 10 CUs, you will end up with about 12GB added to the system drive and about 1.5GB added to your other drive.
My preference is to make the ssytem drive large enough to hold the SQL binaries. Even with everything on the system drive, we very rarely see read IOPS go into double-digits, so once the server is booted then system drive performance is that important.
Original author: SQL Server FineBuild 1-click install and best practice configuration of SQL Server 2017 2016, 2014, 2012, 2008 R2, 2008 and 2005. 1 Dec 2016
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Disclaimer: All information provided is a personal opinion that may not match reality.
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