I've been using LineageOS 14.1 on a rooted unlocked Samsung S7 (G930F) for a long time. My phone started dying, so I rebooted to TWRP and managed to take a full backup to the SD card before it died. Because TWRP doesn't have an option to include internal user storage files, I lost that partition but it didn't have much I missed - the backup contains my chats, contacts, and apps+data, and DCIM (pics) was being saved on the SD card anyway.
I bought a replacement phone of the identical model, wiped it, unlocked it, transferred over the SD card and SIM cards, installed TWRP, then used that to wipe system+data+cache, and restore all my saved partitions (except boot+baseband+EFS) to the new phone.
It worked fine, but I've found that programs such Titanium and FX file manager can't write to the sdcard. I suspect some files can't be read either, as am attempt to copy a 10GB backup folder to internal storage (18G free) failed for unknown reasons after about 6GB in FX.
So there is clearly some kind of permissions issue going on. But what?
I know the rough scenario of sd card permission changes that came in at the time of KitKat, but that knowledge hasn't helped me resolve it either.
Unlike most scenarios, I've still got the identical combination of system partition + sd card, so unless the file permissions are also locked to say, IMEI or something else equally unlikely, I'm confused why it's having such a problem, and how to resolve it.