It's usually best to install the OS on a reasonably sized partition on a large drive.
The idea is to isolate the OS from the Data because OS read/write tends to churn things up and data layed into an OS makes the problem worse.
Though some fragmentation is normal and unavoidable more than that IS a problem.
however on newer larger drives typically created as "dynamic" drives require conversion
to "Basic drives" to allow. partitioning of the drives.
My 750's are all dynamic "simple volumes"