Is it possible to copy from SD card reader to USB memory stick directly?

Question

I'm looking for an easy way to backup SD cards full of photos on a trip. I have a 3-port USB hub, and can mount an SD card reader to copy photos to my phone running Cyanogenmod. But if I hook up the USB memory stick to the same hub, it doesn't show up so I can't do the backup from reader to the stick.

The workaround would be to copy the contents of the SD card to phone memory, remove the reader, replacing with the USB stick, and copy from phone memory to the stick. But this is cumbersome as my phone has little memory free, so would have to do multiple passes which is slow and (human) error prone.

I also have a Raspberry Pi Zero running an rsync backup script to do the copying, but the script doesn't work consistently or reliably.

So, is there a way I can copy directly from SD card reader to a USB memory stick?

Answer 1

Found a solution after further research.

Install app USB OTG Helper which requires BusyBox to be installed. I believe both apps require root to function properly. After that I attach an unpowered USB hub with multiple ports. Connect the card reader, and tap on "mount" in USB OTG Helper. Connect USB memory stick, again tap on "mount". Then I can copy directly from the card reader to the stick using any file manager.

Note that the mount points the Helper app shows are incorrect. It claims both drives have been mounted under /storage/UsbOtgDrives, but one of them was under usbdisk instead.