Diagnosis
The HomeBase 3 is picky about the microSD card's format and type. Cards that aren't formatted to the correct file system (exFAT or FAT32), are too slow, or are simply defective will cause this error.
How to Fix It
Start with the card itself. You need a high-endurance, Class 10 or U3, A2-grade microSD card, 16GB to 512GB. Once you have a compatible card, format it on your computer first. 1) Insert the card into a computer (using an adapter). 2) Find the drive, right-click, and select "Format." 3) Choose exFAT as the file system (or FAT32 if the card is 32GB or smaller). 4) Click "Format." After it's done, safely eject it, insert it into the powered-off HomeBase 3, power the HomeBase back on, and try formatting again through the eufy app.
If That Didn't Work
The issue could be with the HomeBase 3's own software. Try resetting the HomeBase's network settings. On the back of the HomeBase, press and hold the sync button for 10 seconds until you hear a prompt. This won't delete your cameras, but it will reset network and some accessory settings. Then try the SD card format process in the app once more.
Heads Up
Never remove the microSD card while the HomeBase 3 is powered on. Always go into the eufy app settings for the HomeBase and select "Eject SD Card" first, wait for confirmation, then power down the HomeBase before physically removing the card to avoid data corruption.