Playing video displaying subs from a USB stick/drive...

DVD & Blu-ray subs are are graphics rather than text, though DVDs may have CC embedded in the mpg2 video as well. It is possible to include Blu-ray subtitle streams in a MKV container, but playback options are limited -- I assume there's a way to include DVD sub streams too, but I've never seen or tried it. If you don't alter the original mpg2 video on a DVD with CC, a Few players can display embedded CC. The simplest way is to just copy the DVD or Blu-ray to the USB stick/drive, and use VLC [available for just about any device, including Kindle HD (!)] Otherwise, once you've copied the DVD or Blu-ray to your HDD [to remove DRM], the most common way to do it is to OCR the subs into a text-based .srt file. There are a few free apps to do just that, e.g. SubtitleEdit. If that .srt file's name matches the video file, e.g. MyVideo.mp4 & MyVideo.srt, & both are in the same folder, most players, including native players on Android devices, will let you turn sub display on/off. Because the graphics-based sub streams are contained in DVD VOB or Blu-ray .m2ts files, you can also create a new DVD or Blu-ray re-using the original subs streams/files, or just put the re-encoding [converted] video in a .m2ts file along with the original subs, but you have wider player compatibility using .srt files.