Is it possible to use usb drive/sd card as internal storage? Phone internal storage is damaged and unavailable. Is there any other way to fix the phone than replacing internal storage (or entire motherboard), perhaps using usb drive or sd card?
A feature called Adoptable Storage allows the Android OS to format an external storage media as a permanent internal storage. The data on the adopted SD card is encrypted and it can’t be mounted on another device.

Here are the steps to adopt your SD card:

Put the SD card inside your Android phone and wait for it to get detected.
Now, open Settings.
Scroll down and go to the Storage section.
Tap the name of your SD card.
Tap the three vertical dots on the top right corner of the screen.
Tap Storage Settings.
Choose format as internal option.
On next screen, you have the final chance to decide whether you want to change your mind. Tap Erase &amp; Format if you want to format your SD card as internal storage.
You will be notified if Android finds that your SD card is slow. Tap OK to continue.
You can start the data migration process now or do it a later stage.
Tap Done to finish storage adoption process for your SD card.

Source: https://fossbytes.com/android-sd-card-internal-storage-adoptable-storage/