How to restore visibility of SD card on mobile phone?

An "unknown error" can be either the destruction of the file structure or a hardware malfunction of the memory itself. To begin with, it is worth clarifying whether you need to restore the visibility of the data on the map or if you just want to continue using it - your further actions will depend on this.

If the data from the card is not needed

In this case, you just need to format the card through the settings and continue to use it.

If the card is no longer visible in the settings, then it must be removed from the phone and tested on a computer through a card reader. To do this, you can use any program for checking disks for bad blocks - for example, HDD Scan ("read" mode).

If the card contains unreadable cells or is not detected at all, then it is considered unusable. It remains to either throw it away or return it under warranty.

If the data is needed and the memory card is working

There is an important point about how the card was originally formatted - as a removable storage device for data transfer or for internal use as additional phone memory.

Previously, when Android did not use encryption by default, working flash drives could be scanned by data recovery software.

However, now, when encryption is already used in every phone, when formatting a microSD, in most cases, information is also encrypted (depending on the manufacturer of the phone). And as a result, nothing can be restored on the computer on its own, since the encryption key is in the smartphone.

If you don't know if you have encryption or not, just try any one for data recovery. If the memory card is encrypted, nothing will be found on it.

If the card was used as additional memory, then it will be encrypted. In this case, nothing can be done at home, since specialized software is required. The authorities (Ministry of Internal Affairs, FSB) and some companies that are engaged in data recovery have it. But it's not cheap.

If data is needed and the memory card does not work

In cases where the memory card is faulty, you need to solder to it, read it with a programmer and disassemble the resulting image, since the data does not lie there explicitly, but is scattered according to a certain algorithm.

All these manipulations also require specialized equipment and software, and sometimes even an X-ray to figure out which contacts to connect to.

And this means that in such a situation, data cannot be restored on its own - you will have to give the phone for a few days to a laboratory that is engaged in data recovery.