?? ExtHD OS admin-acct can access user files on Mac boot device!

Eii! Surprising and welcome revelation is that when booted into OSX 10.9.5 on an external drive, I can (when in admin acct in that OS) access & copy data in a standard-user's account residing in the OSX 10.9.5 OS of the boot device (both a MBP & a Mac Mini).

How is this possible? All the aggravation a Mac puts one through with discrete user accounts and yet, I can breeze right into a standard-user account's data residing on the boot device.

The external drive OS first used was cloned from a Mac HD. Theorizing that might be why I could so readily access a user's file on the boot device (even tho acct names were different), I did a fresh install of 10.9.5 (from thumb installer) onto the external drive and set up admin & std accounts with totally different names. Made no difference.

I found that only the admin acct on the external drive's OS (into which I had booted from an initially fully-powered-off Mac) could read data in a standard-user's account. However, when signed into a standard account in the booted up external drive's OS, I could still readily access the underlying MacHD system and library files.

I *lock* up whereever I can, so I don't have things such as automatic login enabled. Everything is passwroded up the wazoo. Frankly, given the headaches with user file ownsership of having each user account so locked up, I am really ticked off that I'm seeing wide-open user files and ready access to the HD of the Mac used for booting into external drive.

Is this normal behavior? And is there anyway to close the gate more between the two devices in terms of file accessf from the external drive?

I ask this because I had (sigh, naively, it appears) hoped to use the external drive's OS for surfing internet only -- no data or sensitive info on that drive. My hope was that if again caught by malware, it would be less likely to infect my Mac,and I could wipe the external drive (having so little data invested in it).