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My Android is unencrypted and running Android 5.1. However when I restart the phone I cannot access the file system using Android File Transfer using a trusted computer unless I enter my password.

Given that my device has not enabled whole disk encryption what is stopping me from gaining access?
Do I still need to encrypt my files?
If the files are accessible how can I access them in order to test if the encryption I intend to add works?

Note: Encryption is not on by default on Android phones so I'm taking these measures to encrypt at the file level to secure the data for users who have not enabled whole disk encryption on their phone.

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    Is the phone locked? Even without encryption enabled, Android tends to prevent access to storage whilst the device is locked. It will also block access to some folders within the system - these can be accessed by having root access. – Matthew Sep 16 '16 at 15:01
  • Which phone do you use ? Samsung with Knox enabled, Xiaomi etc., Is your device rooted ? – Surendra Patil Sep 16 '16 at 15:11
  • What exactly do you mean by "Android File Transfer"? – CommonsWare Sep 16 '16 at 15:15
  • @CommonsWare It's a program that let's me access the Android file system via my Mac, probably not worth mentioning mind you as it's just an OS compatibility tool. – Declan McKenna Sep 16 '16 at 15:19
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    @crovers That makes a lot of sense. Can you throw that in as an answer, maybe explain a bit about how the data would be accessed? I'll mark it as correct as you've certainly answered my question. – Declan McKenna Sep 19 '16 at 15:35
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When you attempt to access files through a USB port against a running phone, that access is mediated by the running OS. That OS has rules about what can grant access - and one of those rules is (generally) that the phone must be unlocked (and often specifically configured to allow USB file access).

This is because the phone OS is designed to protect the legitimate owner and their files - they have no reason to allow the OS to provide the files unless you can demonstrate you are legitimate (by unlocking / configuring).

Encryption is intended for a separate purpose - for preventing you from doing an end-run around the OS. The data is, after all, stored on NVRAM (or equivalent - not an expert on phone storage) - for someone willing to directly access this memory using hardware (or for someone able to root the phone without clearing the storage), the OS protection is nullified (since it either isn't running (hardware attack) or is under control of the attacker (root))

File System Encryption prevents access to the data even if the user can access the raw bytes on the file system - without the key, that access becomes useless.

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