I've seen both these forms used and I was wondering, is either of them incorrect or are they interchangeable?
You can download something ON your device.
You can download something ONTO your device.
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The difference is that on denotes the device I'm interacting with, and onto denotes the destination for the received data. The confusion may arise because in the first case below (interacting with the destination device), the two are implicitly the same.
I use a device directly, initiating a download using its own interface or controls, I am downloading on it. That download is almost certainly being stored onto the same device where I initiate it.
I attach a device to a PC, and initiate a download on the PC to be stored on the attached device, I am downloading on the PC and storing it onto the device.
The act of downloading is to effectively copy a file from one place (normally on the internet somewhere) to your local machine. In this case, if you're saving the file on your desktop for example, the correct usage would be ONTO as that refers to the location of where you are downloading it to.
However, you are using the to device to download... to technically you are downloading on a device too. I would always use ONTO.
Would it work if you were referring to uploading? "You can upload something on your device" vs. "You can upload something onto your device".