[Xiaomi Mi Max Questions & Answers] E90 Commie:
I decided to start a new thread about this since I have been searching high and low in the forum about this particular issue. I copy and paste my post I wrote in the RR thread since I think it is worth to bring it to more public attention.

I also found some comments on this very issue in the Redmi Note 3 Kenzo forum about Nougat/RR 5.8.2 and in other forums (about nVidia Shield) as well. The basic issue is about app access (Kodi for example) to USB-OTG flash drives. It also affect MX Player and FX File Explorer. Here is my earlier post:

There is one thing I would like to point out - perhaps a solution can be found:

App access to external USB flash drives

Nougat and some ROM/kernel combinations seems to act differently than Marshmallow in this regard - it cause some issues with Kodi (as one example) and also some file managers (FX File Explorer). The symptoms are:

1. USB flash drives doesn't show up as 'regular' storage volumes in FX (the app demand the user to 'connect to the drive' and insist on transferring every file in order to view them - videos is a typical example). It happens with Root Explorer as well in some cases.

2. Kodi is unable to access a USB flash drive. It can navigate the file tree down to the /mnt/media_rw but is unable to access the drive itself (i.e. nothing happens when selecting media_rw).

3. StickMount doesn't help either - the USB disk remains inaccessible for Kodi and FX.

Apps such as MiXplorer and Files (the native file manager, not the CM one) is able to manage the USB disk properly.

The ajmsg78 packaged RR 5.8.1 with Nebula v1 seems to operate similar to Marshmallow in this regard - FX File Explorer and Kodi are able to treat a USB disk in the same way as a memory card (browse files, selecting a folder as a Kodi source etc).

It only works on that particular ROM/kernel combo - flashing Nebula v2 or Harfix R2 immediately change the storage access (i.e. FX want to 'connect to storage' and Kodi loose the ability to access the disk).

I think the main issue is the way Nougat handles app access to those disks. There are no problems with accessing the regular microSD card - Kodi and FX are able to handle it perfectly well. It is only when a USB flash drive is connected that the problem arise.

The easiest way of checking it is:

Install FX File Explorer and/or Kodi. Connect a flash drive and see if it shows up in FX and/or become accessible in the Kodi file browser. If FX want to 'connect' to storage instead of simply mounting it or Kodi is unable to browse it - then the issue is present.

There are something in the ajmsg78 packaged RR 5.8.1 by marchetto96 with Helium fixes including Nebula v1 that make it work. Flashing another kernel (Nebula v2 or Harfix) revoke the access. StickMount doesn't help. So there are something special about the Nebula v1/RR 5.8.1 combination (I don't think it is the ROM itself but the kernel - flashing that ROM version with Harfix R2 before setting it up means no USB flash drive access).

It would be interesting if a solution can be found. I have been trying the latest Kodi 18 Leia nightlies without success (can't browse the media_rw). SPMC (the Android fork based on Kodi 16) is also unable to get such access.