I followed the various guides and they were pretty close, to get the router to boot from external storage. These are R6350/6850's, one with a USB hub that has an SD card (same function here, as a usb stick). It seems there's marginally less flash wear, if I can boot from and use an external storage device for the flash, and more room for added modules. That is my intent in doing-so.
I am wondering if there is a way to 'flash' only the external storage when there is an update to Openwrt? My guess at this time, is an update will only flash the router internal storage, then that will have to be copied to the SD card, mimicking the initial set up procedures for switching from internal boot storage to external.
It seems you are talking about extroot. For clarity purposes, the router isn't really booting from external storage... it is booting from the internal storage, and then mounts the overlay partition (fairly early in the boot process) which contains all of your configuration files and user-installed packages. In the case of extroot, the overlay is on your external drive. This has a practical effect that appears to be booting from the external media, but actually the booting is happening from the internal flash storage. It's a bit of a pedantic difference in day-to-day use cases, but a very important distinction with respect to the way the process operates at a technical level.
When else would you think that there would be a need to flash/re-write the card?
And what are your concerns?
Data will be written to the card anytime you do the following:
perform the extroot process (i.e. copying the data from the internal storage to the external media) -- this is necessary once per OpenWrt version upgrade (major and minor updates).
Installing/removing packages
Changing configuration files
Writing/deleting any other data files to persistent storage
OpenWrt uses RAM for all running files and logs, so this would only happen if you manually configure the storage for use with the system logs, user files (as a network drive, for example), or other similar purposes.
Although not exactly the same as your question, I talked about what is happening with extroot fairly extensively in this thread: