[Samsung Galaxy A3, A5, A7 (2017) Questions & Answe] shingdong:
As the title suggests, I have successfully installed TWRP, but a problem all along was my phone not being able to mount /data at all (even with the old stock one and so I couldn't ever really do anything with it)

I should probably first start off as to how I got here, as that might be very useful information;

Basically I got my phone from a place that imports from the UAE, with all of its software and everything being from there. So I got tired of it and went ahead and tried to install TWRP then first. It worked, then at the whole "Install app on phone?" screen, I swiped right with all of the boxes ticked (the set as system app and the other one) and my phone went full retard and rebooted itself while also going into the restore mode and it went ahead and forced a full format of the device, but the catch was that it couldn't at all format itself, so it just went back to having the normal everything, with all my apps and data. Soon after, I saw the Software Update on the notification panel pop up and I was curious as to why it wouldn't give me the option to update. I opened the update section and it gave me the "can't update because phone is rooted" deal. I tried installing the update through the Smart Switch app and it was a success. (note that before this, my KNOX was tripped and my warranty showed up as being voided, but after the update, they reverted to their normal unmodified state - which was super confusing for me, I don't know if anyone else had this happen to them).

My thinking here is that for some reason the /data partition went crazy and now its completely inaccessible to anything.

And now, after this, I went ahead and downloaded my country's 8.0.0 update (MBM). It worked fine and everything, the boot logo glitched out a bit, but it would always boot into the normal phone state, of course with everything removed.

A few days later, the UAE Oreo update had come out and I switched to that, it installed like normal and everything was fine. Not too later after this, I encountered an error and I had to factory reset through my recovery mode. Whenever I did do a factory reset, it didn't go back to the UAE 7.0.0, but instead deleted all my data and returned to the 8.0.0 T-Mobile system. Here's another thing though, whenever I did try to enter recovery mode, my phone went bonkers and said "Installing Software Update" and then goes into that "No Command" screen, and when I pressed the same combination for the stock recovery mode, it popped up, but it gave me an error message that it was unable to detect the /data partition or something. I looked it up and every site said that it's nothing to worry about and I quickly brushed it off.

Now comes the bad part.

When I installed the whole TWRP, I changed the /data file type from ext4 to exfat and then back to ext4, which allowed TWRP to detect the /data partition, and with that, I flashed the RMM State Bypass, and wiped the dalvik and regular cache, which to me signified that everything was done correctly. I proceeded to the boot to system and the "Install TWRP app?" screen with all of the boxes ticked by default - I didn't change anything, the phone froze up a second (the console did not move at all after swiping to install the app) and it went to hell when it did boot back into system.

Now my phone is at the

Device storage corrupt

The data partition has been corrupted.

You need to reset your device to

factory default settings. This will erase

all your data.​

and when I do press the reset button, my phone goes back into TWRP to factory reset, but it can't mount the /data partition. To be more precise, this is what it says:

Could not mount /data and unable to find crypto footer.

Failed to mount '/data' (Invalid argument)

Unable to recreate /data/media folder.

Updating partition details...

Failed to mount '/data' (Invalid argument)

...done

Unable to mount storage

Failed to mount '/data' (Invalid argument)

Full SELinux support is present.

CryptKeeper.showFactoryReset() corrupt = 1

Unable to mount /data/media/TWRP/.twrps

Running Recovery Commands

Formatting Cache using make_ext4fs...

Failed to mount '/data' (Invalid argument)

Done processing script file​

Currently as I posted this, the 7.0.0 version is downloading and I'll try to re-install that one, update it to 8.0.0 through Smart Switch again and I'll see if the factory reset goes back to the 7.0.0 or keeps deleting all my data while returning to the 8.0.0.

I'm more than positive that whatever I've done whenever I did it, I must have deleted the factory reset thing and now it's just stuck in a bootloop.

I'm looking forward to any kind of help to get me out of this mess. Thanks in advance. :D