[Android General] pndwal:
Sounds like this may be a problem seeing mtp storage. Using USB mass storage may be needed for this.

As discussed on p33 in this thread, this tablet actually has it in stock ROM, called USB Storage. I could simply plug device into my set top box, and USB Storage option came up (with note win XP is recommended) allowing STB to play MP4 files as it will from a USB stick, and record to SD in device too. Of course, this won't work if SD is formatted adoptive (F2FS and encrypted). Option simply won't show. I think FAT is required too.

I can't see this option with LOS14.1, so it may need enabling as mentioned below.

As has been noted, old bulk storage ability is no longer officially supported in android OS. (This happened due to problems when phone looses access to whole SD in this mode - phone retains access in mtp mode.) However, most Lenovo devices (up to M anyway) seem to have it enabled still.

On devices without out-of-the-box support, it can usually be enabled using Universal USB Mass Storage enabler, here:

https://xdaforums.com/android/apps-games/app-universal-mass-storage-enabler-beta-t3240097

This previously required SELinux set permissive (not sure if that's changed), but it takes care of this itself after requesting permission. However, with stock we have mass storage (with enforcing SELinux) out-of-the-box as I mentioned.

More info on Mass Storage / MTP differences:

Some files not showing up with mt protocol is because phone retains access to filesystem in mtp mode, and may not have handed off access to that file as a phone app or phone system is still accessing it. This solves problems when phone looses access to whole SD in mass storage mode, but can be annoying figuring out why a particular file or folder is not showing.

I also used mass storage for years without problems, but an understanding of the differences in transfer protocols allows you to see the potential problems with mass storage. Probably most disastrous problems occur with PC connection. Windows often detects problems with a 'drive' when there aren't any and may offer to scan and fix for example, and I've seen this corrupt mass storage, so you can appreciate concerns many (like [MENTION=7383447]MotoJunkie01[/MENTION]) have regarding mass / bulk storage. And these problems are eliminated with the selective hand off of access used by MTP (which has the added benefit of still being able to display internal user data partition despite being formatted in faster Ext4 / F2FS etc, as well as SD cards formatted with F2FS, Ie adoptive).

Android policy itself is pretty consistent - only MTP support since 4.x. But OEMs like Lenovo seem to have resisted the change. Perhaps they see the benefits of supporting mass storage as others do, and kept it alive at least in K, L & M. - (this may only be on MediaTek devices.)

With the new adoptive formatting ability (most with low end speced devices really appreciate this), MTP transfer is just plumb necessary, so my personal feeling is that MTP adoption as an across the board solution is a good decision.

A bit more on why Google did it:

Hope all this helps with your setup / preferred apps. You may be able to find better / newer file sync apps that play nicely with MTP.

Many have faced the similar issues since USB Mass Storage was depreciated, and have reluctantly admitted defeat and adopted alternative methods. However, the fact Lenovo retained kernel support for this (and we use unaltered stock kernel for custom TB3-850F LOS14.1) and have it enabled in stock, bodes well for enabling / using this in TB3-850F LOS14.1.

Please let us know if you find any solution. All best, PW