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Connecting an External Memory to an iPhone or iPad
If you are using a 2018 iPad Pro with a USB-C port and a USB-C flash drive, you only need to use the Plug in the flash drive.
However, if you have another iPad or iPhone, you will need an adapter. You can use a Lightning to USB adapter to connect to any USB flash drive. This also works for a USB SD card reader.
Alternatively, you can purchase Apple's standalone Lightning to SD card reader.
Support for external hard drives or solid-state drives is at best questionable. However, you can connect external USB-C drives directly to your iPad Pro via the USB-C port. Drives that require more power, especially drives with rotating hard drives, require an external power source.
If you connect a drive and the message "This accessory needs too much power" appears, you must use a plug with a power pass-through. You can use these for iPhones and older iPads with a Lightning to USB 3 camera adapter that allows you to power your device (and the drive) through a Lightning connector.
Copying Files from External Storage
We use the Files app to copy data to and from external storage. First, connect your USB flash drive or SD card using one of the methods described above, and then open the Files app.
Go to the Browse tab. If you see your drive in the "Locations" section, tap it to see all the files and folders on your drive.
Tap a file to preview. The file app has a robust media viewer. You can open text and audio files, or even watch videos from your drive without exporting them to an app.
Move some data from your external drive to the local storage of your iPhone or iPad. First, under "Places," touch "On my iPhone" or "On my iPad."
Tap and hold the empty area. and then choose "New Folder".
Name it, and then tap Done to create a new folder for the files you want to transfer.
Now select your USB flash drive in the Locations section.
From the menu, tap "Select" on the top toolbar, and then select the files you want to move.
Tap Move on the bottom toolbar.
This opens a menu of all available locations in the Files app. Touch "On my iPhone" or "On my iPad" to expand the local storage option. Select the folder you just created and tap "Copy". The files are transferred to the folder.
CONNECTION: Anything you can do with the file app on your iPhone or iPad
Copy from Files to External Storage
After You Have Connected To your external drive, you can copy data from any location in the Files app as described above. This means that you can copy files from your iCloud drive, Dropbox, a connected SMB server, or any app that you add to the "Locations" section of the File app. Let's say you want to put photos on a USB drive export. First, go to the photo app, select the images, and then tap the Share button.
Select "Save to Files".
This invokes the versatile file extension that lets you save files to any location, including attached external drives.
Tap on your external drive and then select a folder. You can also tap the New Folder button at the top to create one. Select the folder on the external drive and tap Save.
. In a second step, your photos are selected in full resolution in the folder you selected on the external storage device. To confirm this, go to the Files app and navigate to the folder.
The app "Photos" is just one example. Use this method to move PDFs, videos, images, and more from an app to external storage.
Using the File App
What if the file is already stored in the file app? With the drag and drop function you can move the files quickly.
Open the Files app and navigate to the folder where you saved the files. Next, tap and hold a file and slide your finger away from the icon. You should now be in file selection mode.
If you want to select multiple files, tap on them with the other finger. 
Then use the other hand to switch from the Locations menu to the external drive. Continue to hold the files with one finger and navigate to the folder where you want to save them.
When you reach the destination folder, you can share the files and they will be copied to external storage.
Done! You can now remove your USB drive from your iPhone or iPad. You do not have to throw it out for sure.
Acting as File Owner
As you may have guessed, the file app is quite complex, and there are more than a few ways to perform a single action. The best way to become a Files Power user is to explore it, as the version in iPadOS 13 helps turn your iPad into a real computer.
After the upgrade you should first activate the Dark mode.  RELATED: iPadOS turns your iPad into a real computer