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TLDR (too long, didn't read): Hand movements could lead to disconnection, which could lead to data loss. Don't hold your phone in your hand while using OTG. Even better: avoid OTG altogether and use MicroSD or move files using an external Windows or Linux computer.
When moving files to a USB stick connected via USB-OTG (on-the-go, meaning the USB stick connected to charging port through an adapter) to free up space on your phone, do not hold the phone in your hands. Your hand movements could lead to the disconnection of the USB OTG adapter. A file might be deleted on your phone before it was fully written to USB OTG.
Put the phone on a table or on the ground and leave it there until the file transfer is finished and you have unmounted the USB stick.
Another possibility is copying files, comparing the size and the number of files, and if they match, delete the files from the phone to free up space. The risk with that wrong files are selected in the deletion step. When you move files, you know that the same files you selected are deleted from the source device. But if you copy and then delete
When moving out files, the file manager might think a file has been moved out already and deletes it from the source device. But parts of the file are still in the writing cache
Connect the phone using a USB cable and a USB stick on a different port. Avoid using USB hubs because they add weak spots that could disconnect too. (This is why the trend of laptops having fewer USB ports for "oh-so-shiny minimalism" is a bad thing. I call it toxic minimalism. Sacrificing practicality to follow a design trend. Hubs are not as good as built-in USB ports. If your laptop lacks the built-in USB ports, move the data to the laptop's internal storage first.)
However, moving files using a USB cable has a different problem: due to bugs in the Media Transfer Protocol (MTP), some files might not appear. For this reason, you must always move files and not folders. Moving folders from an MTP device, your smartphone, is unsafe because your Windows/Linux file manager might think a directory is empty and deletes it even though it was not empty, but not all files were shown by MTP.
[I hereby release this post under Creative Commons 4.0 Attribution ShareAlike. Let's get the word out there.]
When moving files to a USB stick connected via USB-OTG (on-the-go, meaning the USB stick connected to charging port through an adapter) to free up space on your phone, do not hold the phone in your hands. Your hand movements could lead to the disconnection of the USB OTG adapter. A file might be deleted on your phone before it was fully written to USB OTG.
Put the phone on a table or on the ground and leave it there until the file transfer is finished and you have unmounted the USB stick.
Two weak spots
USB-OTG (on-the-go) has two weak spots. the junction between the USB OTG adapter and the charging port, and between the USB stick to the USB adapter. If any of them disconnects while writing data, data could be lost. Even minor hand movements could lead to a temporary disconnection, and if that happens while data is being written, data could be lost.Another possibility is copying files, comparing the size and the number of files, and if they match, delete the files from the phone to free up space. The risk with that wrong files are selected in the deletion step. When you move files, you know that the same files you selected are deleted from the source device. But if you copy and then delete
Technical details
Operating systems such as Android and Linux use write caching to improve performance and prevent lagging. However, if the data storage device is disconnected before the cached data is "flushed", meaning written to the device, any unwritten data is lost. This is also the reason the "safe unmounting" feature exists in operating systems, under various names.When moving out files, the file manager might think a file has been moved out already and deletes it from the source device. But parts of the file are still in the writing cache
Even better
Move files through a desktop computer or laptop running Windows or Linux. Desktop computers and laptops have a more stable power supply than a little phone battery.Connect the phone using a USB cable and a USB stick on a different port. Avoid using USB hubs because they add weak spots that could disconnect too. (This is why the trend of laptops having fewer USB ports for "oh-so-shiny minimalism" is a bad thing. I call it toxic minimalism. Sacrificing practicality to follow a design trend. Hubs are not as good as built-in USB ports. If your laptop lacks the built-in USB ports, move the data to the laptop's internal storage first.)
However, moving files using a USB cable has a different problem: due to bugs in the Media Transfer Protocol (MTP), some files might not appear. For this reason, you must always move files and not folders. Moving folders from an MTP device, your smartphone, is unsafe because your Windows/Linux file manager might think a directory is empty and deletes it even though it was not empty, but not all files were shown by MTP.
MicroSD
Or move the files to the MicroSD card inside your phone (See? Memory cards are not obsolete and will never be, to all those "internal storage is enough" preachers.) and access it on a computer from the MicroSD card. You can also swap the MicroSD card to have immediate free space. The MicroSD card is inside a stable placeholder inside the phone and will not disconnect from hand movements, whereas the USB-OTG connection is fragile and subject to disconnection.[I hereby release this post under Creative Commons 4.0 Attribution ShareAlike. Let's get the word out there.]
