> - Almost at the very beginning, there was no /usr folder in UNIX. All binaries were located in /bin and /sbin. /usr was the place for user home directories (there was no /home). Putting /home on a separate partition remains a pretty common thing to this day because users will tend to have greater storage requirements than just the root. /usr/bin and the like are the result of people realizing that this secondary larger disk is an acceptable place to put binaries and other files that aren't needed at bootup.
The author cites to this post by Rob Landley – http://lists.busybox.net/pipermail/busybox/2010-December/074...
While I can't independently confirm Rob's claims, and he doesn't provide any citations, I do find them very believable – /usr was invented at Bell Labs because they were running out of space on their puny 1970s hard disks. (And an RK05 was small even by 1970s standards – the IBM 2314 mainframe hard disk, released in 1965, had a 30MB capacity; the IBM 3330, released in 1970, stored 100MB – of course, these disks would have cost a heck of a lot more than an RK05, and were likely not feasible for the UNIX team given their budget.)
If they had bigger disks (or the facility to make multiple smaller disks appear like one big disk) – it is less likely they would have split the operating system itself across two disks (/ and /usr). (Using separate disks for the OS vs user data was more likely even with bigger disks since that was common practice on systems at the time.)
(Some other operating systems from the same time period already had some ability to make multiple disks appear like one big disk. For example, OS/360 has the concept of a "catalog", which is a directory mapping file names to disk volume names; this means you can move individual files between disks without changing the names by which users access them. In their quest for simplicity, Thompson and Ritchie and co decided to omit such a feature from UNIX.)