Sync ext SD to USB OTG **without** using a PC

**Device Details** * Moto X Play - Android 6.0.1, Rooted (Systemless root) * Samsung 64 GB ext SD * [San Disk 128 GB USB](http://www.amazon.in/SanDisk-SDCZ43-128G-G46-128GB-Ultra-Flash/dp/B00YFI1EBC) - came shipped with FAT 32 reformatted to exFAT since I have folders > 4 GB on external SD **Use Case** Sync **entire contents of ext SD to USB (one way sync without deletion of SD card contents )** on weekly schedule / on demand , as [incremental backup](https://en.wikipedia.org/wiki/Incremental_backup) **without** using a PC ( I do have one but reasons for not wanting to use it are not relevant) **Other Details** * USB drive recognised by device correctly connected by standard OTG (*Settings → Storage & USB* ) * [Busybox](https://play.google.com/store/apps/details?id=com.jrummy.busybox.installer) installed . It came bundled with ROM Toolbox Pro. I haven't used it directly. If another busy box is to be installed, it's fine as long it doesn't clash * Mount points as below **Things tried** * Unsuccessful app search- I wasn't able to find an app that can do this ([Folder Sync](https://play.google.com/store/apps/details?id=org.t2k269.foldersync) claims to but it can't see USB) * Googling for alternatives * MacroDroid automation fails, since app can't see USB (raised this in app forum) **Question:** How can I do this ? I am open to **any means** Please note that I intend placing a **bounty of 100** , any answer meeting needs before I post bounty will be awarded similarly (I may delay posting the bounty to ensure I get a solution) Please **avoid** solutions that * are not tested - it should have been tested on your device and USB configuration to give it credibility * Involve PC , uploading to cloud, transferring to remote server etc * References to claims from doubtful sources which can't be verified * Involving use of File Managers ( I had mixed results with these but then that is for another question) Aim of the above is not to deter solutions but to discourage frivolous attempts. If clarity is needed, please ask I discussed this need on chat and some potential candidates for solution included *BusyBox, rsync and Tasker*. I know **nothing** of these means so **Please do provide complete details**. Being a bounty question (almost), I expect that I should be able to straightaway implement successfully without seeking clarifications. ( Being a rooted device with systemless root, solutions using root are fine. However, if OS is to be modified on any way, please highlight as systemless doesn't permit it, though there is a work around ) I am strapped for time and need to get this working ASAP