Single post / Thread: TL3/626 Can't write on internal memory
Ok, I solved my problem.

Firstly: "sw_20170718_626_W_5.14_1225_5.12_CI_gdb_pn_630_us er" didn't work for me – infinite hourglass when booting. I mean process of patching monitor.app and adding original serial text file didn't work. BUT with "sw_20160922_626_W_5.14_791_5.12_CI_gdb_pn_630_use r" same process worked perfectly.

Secondly: as I used Dropbox to sync my books, I wanted an update (as only from version "W_5.14_1225" updates to Dropbox API were included). So I started researching more and I found that there is a way to "rebind" any Pocketbook to any SD card.

Here is the link to forum post (in Russian): link.
Here is the Google translation: link.

Basically you can download an app (linked in the above post; reposting here as well: https://yadi.sk/d/bo7FNPlRy_fMzA ) and then use it to regenerate ".freezestatus" file (replace existing one with the new one at "/mnt/sde9/.freezestatus") with a command:
After this it just works WITHOUT replacing monitor.app and so on.

P. S. Russian post includes much more details and more usefull info, but for basic "clone the SD content from original to new one and make it work", this replacement of ".freezestatus" is enough.