The SD card not showing up on your phone issue happens due to various reasons, for example, loose connection, file system incompatibility, SD card corruption, and so on. According to the reasons, different solutions are needed here. You can check the connection, format the SD card to give it a compatible file system, or repair your SD card.
Solution 1: Check the connection
You can bridge the SD card and your mobile phone with an alternative SD card adapter or reader, or you can insert the SD card into other devices to test if the SD card shows up, or you can connect another SD card to check if your mobile phone can read any other storage media.
Solution 2: Format the SD card
If you ensure that the connection goes well, then let's focus on the file system compatibility. Mobile phone is able to recognize FAT32, Ext3, and Ext4 file system formats. If your SD card is formatted as non-FAT32, non-Ext3, or Ext4, then it's likely not showing up on your phone. You can format your SD card on Windows to give it a phone-compatible file system.
Before formatting your SD card, make sure you have made a full backup of all files and documents on it. Formatting will wipe out all the things on a hard drive.
Step 1: Double-click 'My Computer' (or 'This Computer') on the desktop to open it.
Step 2: Select your SD card shown on the panel and right-click it.
Step 3: Click the Format... option, and then a Format panel will pop up.
Step 4: Select FAT32 in the file system box, and click Start. Then, wait for your SD card to be formatted.
Solution 3: Run CHKDSK to repair your SD card
If the SD card file system can be recognized by your phone but still not showing up, I guess you should run CHKDSK to repair your hard drive instead.
Step 1: Type 'cmd' in the Search Box and the Command Prompt will pop up. Then, right-click it and select Run as administrator.
Step 2: Type the command as below (You can also use parameter /r, and e: means your drive letter) and press Enter.
chkdsk /f e:
Step 3: Waiting for the CHKDSK stages to finish.
Hope my answer can help you out of trouble!
For more issues about the SD card not showing up, please read the relevant articles: