A first backup of a whole photo library can take a long time — and the longer it runs, the more chances something has to interrupt it. You come back to find it stopped at 60%, and two questions follow immediately: what happened to the files it already copied, and do you have to start over?
Here's what causes a phone-to-USB backup to stop, what it means for your files, and how to pick up where it left off.
Why phone-to-USB backups stop partway
1. The drive lost power or connection
This is the most common cause by far.
- Portable hard drives — the kind with spinning disks — often need more power than a phone supplies. They mount, start copying, then drop out.
- A bumped cable or a worn adapter breaks the connection for a split second, which is all it takes.
- On OPPO, Realme, and OnePlus phones, an OTG setting can switch itself off after about ten minutes without activity. (Android phone not detecting your USB drive? covers this.)
2. The phone suspended the app
Phones pause apps that aren't on screen. On iPhone, an app that leaves the foreground — because you switched apps or the screen locked — is suspended shortly afterwards, which can stop whatever it was copying.
3. The drive filled up
A library is usually bigger than people expect, especially with 4K video. When the drive runs out of space, copying stops.
4. The battery ran low
A long backup uses battery. If the phone shuts down or enters a low-power state partway through, the backup stops with it.
5. One file couldn't be copied
Sometimes a backup doesn't stop at all — a single file fails, such as a long video on a FAT32 drive, and a manual copy may give up there. Why some photos fail to copy to a USB drive covers individual failures.
What happens to the files already copied?
Files that finished copying are fine. They're on the drive, complete.
The file being written at the moment of interruption is the one to worry about. It may be incomplete — and with a manual copy, it can look like a real file in the folder even though it isn't whole.
And if the drive was pulled out in the middle of writing, the drive's own folder structure can occasionally be damaged, because neither exFAT nor FAT32 is designed to survive being unplugged mid-write. If a drive behaves strangely after an interruption, check it on a computer.
Can you resume without starting over?
With a manual copy, generally not. If you were copying with the Files app or a file manager, you have to work out how far it got and copy the rest yourself. Copying the whole folder again can create duplicates of files that were already there, and an incomplete file from the interruption may still be sitting among them.
With VaultDrive, yes. It keeps a record of every item in the backup, so an interruption costs you a pause, not the run:
- It picks up where it stopped. Files already on the drive are skipped, and the rest — including whatever was in progress — are copied.
- No re-copying. Resuming skips files that are already on the drive — on the free version too, not just Pro.
- An unfinished file never counts. Every file is checked against the original after copying, so a file that didn't finish is never treated as backed up — it's copied again.
- It survives the app being closed. Even if the app is killed or the phone restarts mid-backup, the items that didn't finish are picked up again when you resume.
How to resume with VaultDrive
If the drive disconnected: reconnect it. On Android, VaultDrive tells you the drive was disconnected and offers Reconnect & Resume. On iPhone, open the app and resume the interrupted backup.
If the phone left the app: on iPhone with iOS 26 and later, the backup keeps running after you switch apps, with progress in a system Live Activity — though iOS can still end it early. On earlier iOS versions it pauses about 30 seconds after you leave, and resumes automatically when you come back. On Android, the backup keeps running in the background with the screen off.
If the drive filled up: VaultDrive stops the session rather than failing file after file. Free up space on the drive or switch to a bigger one, then resume — it continues with what's missing.
How to prevent interruptions
- Use a portable SSD, not a hard drive. SSDs need less power, have no moving parts, and are faster.
- If you must use a hard drive, power it with a powered USB hub or its own power supply.
- Use a good cable or adapter, and put the phone somewhere it won't get knocked.
- Plug the phone in. VaultDrive also shows a hint if the phone is unplugged with less than 20% battery.
- Check free space first. VaultDrive checks before it starts and won't begin a backup that wouldn't fit.
- Let the screen stay on. VaultDrive keeps the screen awake and dims it while a backup runs, so the phone doesn't auto-lock and suspend the app.
- On OPPO, Realme, or OnePlus phones, make sure the OTG setting is on.
- Do the big first backup at home, overnight, plugged in. After that, backups with VaultDrive Pro copy only what's new, so they're short enough that interruptions rarely matter.
How VaultDrive handles a long backup
VaultDrive is built for exactly the kind of long, first-time backup that's most likely to be interrupted:
- Resumes instead of restarting, skipping everything already on the drive — on the free version too.
- Reconnect & Resume on Android when the drive drops out, with the backup running in the background and progress, pause, and cancel in a notification.
- Keeps going on iOS 26 and later after you leave the app, and keeps the screen awake while it's open.
- Checks every file against the original's size, with an optional SHA-256 check, so nothing incomplete is ever counted as backed up.
- One problem file doesn't stop the run. It's logged with a reason, and the backup carries on.
- Checks the drive first, warning about FAT32 and stopping a backup that wouldn't fit.
It's free for up to 200 files or 2 GB per backup, and Pro is a one-time purchase that makes later backups copy only what's new.
Frequently Asked Questions
Why did my phone backup to USB stop halfway?
The most common causes are the drive losing power or connection, the phone suspending the app after you switched away or the screen locked, the drive running out of space, and the battery running low. A portable hard drive that needs more power than the phone supplies, or a cable that gets bumped, is the single most frequent culprit.
What happens if a USB drive disconnects during a phone backup?
Files that finished copying before the disconnect stay on the drive. The file being written at that moment may be incomplete. With a manual copy, it's up to you to work out which files made it. VaultDrive never counts an unfinished file as backed up; it copies it again when you reconnect and resume.
Can I resume a phone backup without starting over?
With a manual copy in the Files app or a file manager, generally not: you have to work out where it stopped and copy the rest yourself. VaultDrive keeps a record of every item, so when you resume an interrupted backup it skips what is already on the drive and continues with the rest, including items that were in progress when it stopped. That works on the free version as well as Pro.
Will resuming a backup copy everything again?
Not with VaultDrive. Resuming an interrupted backup continues from where it stopped, on the free version as well as Pro, without re-copying files already on the drive. With a manual copy, copying the folder again can create second copies of files that were already there. For new backups after that, VaultDrive Pro copies only what's new.
Do I have to keep VaultDrive open during a backup?
On Android, no: the backup runs in the background with the screen off. On iPhone with iOS 26 and later, a backup keeps running after you switch to another app, although iOS can still end it early. On earlier iOS versions it pauses shortly after you leave and resumes when you return. VaultDrive keeps the screen awake while a backup runs, so the phone won't auto-lock.
How do I stop a long backup from being interrupted?
Use a portable SSD rather than a hard drive, or power a hard drive through a powered hub. Use a good cable, plug the phone into a charger, and make sure the drive has enough free space. On OPPO, Realme, and OnePlus phones, check that the OTG setting hasn't switched itself off.
The short version
Most interrupted backups come down to power or connection, the phone suspending the app, a full drive, or a flat battery. Files that finished copying are safe; the one in progress may not be.
A manual copy leaves you to work out what made it. VaultDrive keeps a record of every item, so you reconnect, resume, and it copies only what's missing — never counting an unfinished file as backed up.