Short answer: for most phone security cameras, keep the SD card as portable storage. Formatting it as internal storage ("adoptable storage") encrypts the card to that one phone, so you can no longer pull the card and read your recordings on a laptop — and if the phone dies, the card goes with it. Adoptable storage only makes sense on a phone whose internal storage is too small to hold your recordings, and only if you have another way to copy video off the phone, such as a USB cable or a file share over your own Wi-Fi.
Budget phones with 16 GB or 32 GB of internal storage are exactly the phones people turn into shop and home cameras. When you put a memory card in one, Android asks a question that looks harmless: use this card as portable storage, or as internal storage? For a phone you use every day, either answer is fine. For a phone that will sit on a shelf recording video for months, the answer decides how you get your footage back, and what a thief gets if they take the card.
This guide explains what each choice really does, using Google's own documentation, and gives you a three-minute test to run before you mount the phone.
What does "format as internal storage" do to an SD card?
Google's Android Help page describes the two choices in four short lines each (Get started with an SD card).
Portable storage:
- "You can save files, like photos and videos, on the SD card."
- "You can't install apps on the SD card."
- "You can transfer the SD card between devices."
Internal storage:
- "You can load files, like photos and videos, on the SD card."
- "You can install apps on the SD card."
- "You can't transfer the SD card between devices."
- "The SD card can be used in addition to your device's storage."
The line that matters for a camera is the third one: you can't transfer the card between devices. The Android Open Source Project explains why. When a card is adopted, "it's formatted and encrypted to only work with a single Android device at a time" (Adoptable storage, AOSP). The keys are made on the phone and kept there: the platform "randomly generates encryption keys for each adopted device and stores them on the internal storage of the Android device." And because those keys "should not be extractable from the parent device," the same page says, "the storage device can't be mounted elsewhere."
In plain words: after adoption, the card is a locked box, and the only key is inside the phone.
Google's help page also gives the warning you should read twice: "If you format your SD card, data will be permanently deleted." Both directions erase the card — portable to internal, and internal back to portable.
Why this matters more for a camera phone than a normal phone
There is one more step in the adoption process, and it is the one that changes where your video goes. AOSP says that if you choose to adopt, "the platform offers to migrate the primary shared storage contents (typically mounted at /sdcard) to the newly adopted media, freeing up valuable space on internal storage." Google's help page shows the same moment as a choice between Move content and Move content later.
Primary shared storage is where folders like Movies/ and DCIM/ live. Android's developer guide calls it the volume an app "can read and modify" through MediaStore (Access media files from shared storage). Our app writes its finished recordings into Movies/ through MediaStore — we explained that write path, and why it keeps your files reachable, in how to get recordings off an old-phone security camera without a cloud account.
So on an adopted card with the content moved, three things become true at once:
- Your recordings are physically on the card. Good: a 16 GB phone suddenly has room for days of video instead of hours.
- Your recordings are encrypted to this phone. The "pull the card and read it on a laptop" method stops working. A card reader will see a card it cannot read.
- The phone is now a single point of failure. The keys live on the phone's internal storage. If the phone breaks, is reset, or is lost, it follows from AOSP's description that the card cannot be read anywhere else. Google's help page does not describe any way to recover an adopted card on a different phone.
On a phone you carry, point 3 rarely matters. On a camera phone that sits in heat, dust and power cuts for months, it is the main thing to plan for.
The one real upside: a stolen card is useless to the thief
There is a fair argument for adoption, and it is about theft.
A memory card is the smallest, easiest thing to take from a shop. In our guide to what happens if someone steals the phone that is your shop camera, we warned that a portable card stays readable by whoever holds it — and Google notes that erasing a phone through Find Hub "may not delete SD cards."
An adopted card flips that. AOSP says adoption "effectively makes the adopted media as secure as internal storage." A thief who pulls it out gets a card they cannot read on another device.
But be clear about what that protects. It protects the privacy of your footage. It does not protect the footage itself: if the thief takes the whole phone, the card and the key leave together, and you have nothing. The only setups that keep a copy when the phone is taken are ones that copy video somewhere else — for example, streaming to YouTube Live while the shop is closed, or copying the day's files off at closing time.
Can my phone even do this? Three things to check
1. Does your phone show the option? On phones that support it, Google's instructions are: Settings → Storage → SD card → Menu → Storage settings → Format → Format another way. If you never see an internal-storage option, your phone does not offer it on that card. We are not going to list which models do; check yours.
2. Is your Android version in the gap? AOSP has a specific caution: "On devices running Android 7.0-8.1, file-based encryption (FBE) can't be used together with adoptable storage." On those phones, cards "must be used as traditional storage." The same page says "Devices running Android 9 and higher can use adoptable storage and FBE." Many old phones in drawers run Android 7 or 8, so this matters.
3. Is the card fast enough? When you adopt a card, AOSP says "the platform runs a benchmark and compares its performance against internal storage," and warns you if the card is "significantly slower." It also says manufacturers may go further, "such as rejecting adoption completely if the card is extremely slow." Do not ignore that warning on a camera phone. Continuous video is one of the hardest jobs you can give a memory card; see why the microSD card is the weakest part of a 24/7 phone camera and the storage arithmetic for a 128 GB card. On an adopted card, a dying card is no longer "just the card" — it is now part of the phone's own storage.
AOSP adds one more line worth knowing: only cards "in stable locations, such as a slot inside a battery compartment or behind a protective cover, should be considered for adoption." A card in a tray that gets bumped or pulled out is exactly the case it warns about.
The catch with portable cards: does your recording even reach the card?
Portable storage sounds like the safe choice, and for retrieval it is. But there is a detail many people miss.
A portable card is not where apps write by default. Android's storage documentation treats the card as secondary external storage. AOSP says apps "must not be allowed to write to secondary external storage devices, except in their package-specific directories" (Traditional storage, AOSP). And the developer guide's own example for saving a new media file writes to VOLUME_EXTERNAL_PRIMARY — the primary volume — noting that on Android 10 and higher "you can only modify the contents of a volume if it's the primary volume."
So on a phone with a portable card, a recorder that saves into shared folders like Movies/ may be filling up the small internal storage, while the big card sits mostly empty. Whether a given app can put recordings on a portable card depends on that app. Do not assume — test it (below).
This is the real choice, then:
| Your phone | Card as portable | Card as internal (adopted) |
|---|---|---|
| Plenty of internal storage (64 GB+) | Best choice. Recordings stay in internal storage; the card is free for copies and backups you can carry away. | Not needed. You give up card-pull for space you don't need. |
| Small internal storage (16–32 GB), app saves to shared folders | Card may stay empty while internal fills up. Test first. | Shared folders move to the card, so there is room for recordings. You must copy video off by USB or over Wi-Fi — never by pulling the card. |
| Phone on Android 7.0–8.1 with FBE | The only option. | Not available. |
| You worry most about a thief reading your footage | Card is readable by anyone who takes it. | Card is unreadable away from the phone. |
A three-minute test before you mount the phone
Do this once, with the phone still in your hand:
- Check storage. Settings → Storage. Note how much internal space is free, and whether the SD card shows as portable or as part of the phone.
- Record one minute. Start the app, record a short clip with the screen off, then stop.
- Find the clip. Open the phone's Files app and look for the clip on the SD card and in internal storage. Now you know where your recordings actually go — not where you hoped they go.
- Do the math. If the clip is in internal storage, divide the free space by the clip size. That is roughly how many minutes you can keep before old files must be deleted. Our segment rotation write-up shows the arithmetic.
- Practise getting it back. If the card is portable and the clip is on it, put the card in a reader and play the clip. If the card is adopted, copy the clip to a laptop by USB or share it over your Wi-Fi. If the clip only plays on the phone, fix that now — not the day after something happens.
How do I use an old phone as an offline security camera with a small SD card?
You do not need the internet or a cloud account for any part of this. A phone running Background Camera RemoteStream records with the screen off, keeps recordings on the phone, and serves a live view to any browser on the same Wi-Fi through its built-in web server. Nothing goes to a cloud server unless you choose YouTube Live.
That local-only design is why the storage question matters: the phone is the whole system. So:
- Keep the card portable if the phone has enough internal space, or if the test shows recordings land on the card. You keep the simplest recovery method there is.
- Adopt the card only when internal storage is too small, the card passed the benchmark without a warning, it sits behind a cover, and you have practised copying video off without removing it.
- Either way, copy important clips off the phone at the end of the day. A card, adopted or not, is not a backup.
What this guide does not tell you
- Which phone models offer adoptable storage. AOSP describes the platform feature; manufacturers decide whether to show it. We did not test a list of phones.
- How long an adopted card survives continuous recording. AOSP mentions the benchmark and periodic trimming, not wear. Card endurance is a separate question.
- Whether a particular recorder can write to a portable card. That depends on the app. The test above answers it for your phone in a minute.
- Recovery after the phone dies. We found no Google document describing a way to read an adopted card on a different phone. If one exists, it is not on the pages cited here.
Sources
- Android Help — Get started with an SD card (portable vs internal lists, format warnings, "Format another way" steps)
- Android Open Source Project — Adoptable storage (encryption, keys on internal storage, can't be mounted elsewhere, migration of
/sdcard, Android 7.0–8.1 FBE caution, benchmark, stable-location guidance) - Android Open Source Project — Traditional storage (secondary external storage write restriction)
- Android Developers — Access media files from shared storage (storage volumes,
VOLUME_EXTERNAL_PRIMARY)
Background Camera RemoteStream is built by Super Funicular LLC. Get it on Google Play — more guides at superfunicular.com.
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