SD Card - Formatting options

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Under Android, there are two main options for formatting an SD card: as portable storage (mobile storage) or as an internal storage extension (adopted storage). Both options have different characteristics, use cases, and potential disadvantages, particularly when formatting as an internal storage extension.
1. Characteristics of Formatting Options
a) Portable Storage (Mobile Storage)
This option is explicitly recommended

Function: The SD card is treated as a separate, portable storage medium, similar to a USB stick.
Characteristics:
Data such as photos, videos, music, or documents can be manually moved to the SD card.
The card can be easily read on other devices (e.g., PCs, other smartphones) as long as the file system (usually exFAT or FAT32) is compatible.
Apps generally cannot be installed directly on the SD card and remain in the internal storage.
No encryption: Data is accessible to other devices without additional measures.
Advantages:
Flexibility: The card can be easily removed and used in other devices.
No dependency on the Android system of the device.
Lower risk of system issues since the card operates independently of the operating system.
Use Case: Ideal for users who want to store media files or exchange data between devices.

b) Internal Storage Extension (Adopted Storage)
This option is explicitly NOT recommended.

Function: The SD card is integrated as part of the device's internal storage and treated by the Android system as a fixed component.
Characteristics:
The SD card is formatted with a special file system (usually ext4) and encrypted, making it functional only on the device that formatted it.
Apps, app data, and partially system data can be stored directly on the SD card to relieve the internal storage.
Removing the card causes apps or data to become inaccessible since they are tightly linked to the system.
Not readable on other devices due to device-specific encryption.
Advantages:
Expansion of internal storage, particularly useful for devices with limited internal storage.
Seamless integration: Apps and data are treated as if stored in the internal storage.
Use Case: Suitable for devices with limited internal storage when regular removal of the SD card is not planned.

2. Disadvantages of Formatting as Internal Storage Extension
Formatting an SD card as an internal storage extension brings several risks and disadvantages, particularly regarding system instability and data loss:
a) System Instability

Dependency on the SD Card:
Since the SD card is treated as a fixed part of the internal storage, its failure or removal causes system issues. Apps installed on the SD card may crash or fail to start.
A slow or low-quality SD card (e.g., with low read/write speeds) can significantly impact system performance since Android treats the card as internal storage. This leads to delays, stuttering, or app crashes.
Hardware Issues:
SD cards have a limited lifespan (especially with frequent read/write operations). A card failure can destabilize the entire system since Android attempts to access data that is no longer available.
Some devices are not optimally designed for using SD cards as internal storage, which can lead to incompatibilities or unpredictable behavior.
System Updates:
System updates can cause issues when the SD card is used as internal storage. Updates may fail or render the card unusable since Android may not properly process the data structure.

b) Data Loss

Device-Specific Encryption:
The SD card is encrypted and usable only on the device it was formatted on. In case of a smartphone defect, a factory reset, or a switch to a new device, the data on the SD card becomes inaccessible since the encryption is not transferable.
Card Removal:
If the SD card is removed while the device is powered on, data may be lost since Android attempts to access non-existent data. This can also lead to corrupted app data or system errors.
SD Card Failure:
SD cards are less robust than internal storage (e.g., eMMC or UFS). Physical damage, wear, or manufacturing defects can lead to data loss. Since apps and system data are stored directly on the card, these are irretrievably lost unless a backup exists.
Difficult Data Recovery:
Unlike portable storage, where data can often be recovered with standard tools, recovering data from an SD card formatted as internal storage is extremely difficult or impossible due to encryption.

c) Additional Disadvantages

Performance Differences: SD cards are often slower than internal storage. This can affect the performance of apps and the system overall, especially for data-intensive applications.
Limited Flexibility: The SD card can no longer be used for other purposes (e.g., data exchange with other devices) since it is tightly linked to the Android device.
Compatibility Issues: Not all SD cards are suitable for use as internal storage. Android recommends fast cards (e.g., UHS-I or higher), but compatibility issues can still arise.

3. Recommendations

When to Use Internal Storage Extension? Only when internal storage is very limited and the SD card will not be regularly removed. Use a high-quality, fast SD card (e.g., Class 10, UHS-I or better) with a high lifespan (e.g., A2-certified for apps).
Precautions:
Create regular backups since data loss is more likely.
Avoid removing the SD card during operation.
Verify the compatibility of the SD card with your device.
Alternative: Use the SD card as portable storage for media files and move only non-critical data to maintain flexibility and stability.

Conclusion
Formatting as portable storage is more flexible, portable, and safer in terms of system stability and data loss but is not suitable for expanding app storage. The internal storage extension offers seamless storage expansion but carries significant risks such as system instability (due to slow or faulty cards) and data loss (due to encryption, removal, or hardware failure). For most users, portable storage is the safer choice unless internal storage is severely limited and the risks are acceptable.
Sources:
Information based on general knowledge about Android and SD cards.
Parts of the guide were inspired by web sources and personal experience with Android systems.
The guide is provided without warranty!