Do you say IN one of the computers or ON one of the computers? Which is right?
If you hid something like a toothpick inside a computer itself, you would say "The toothpick is in the computer." because the physical object is inside the computer.
When it comes to data or something like that, you refer to it as "The file is on that computer." This is because there's no object that you can hold or touch; you wouldn't reach INTO the computer and pull the file out, you have to use the computer itself to access the data that will show you the file on the screen of the computer.
As a few examples: Data is stored ON the USB drive, and the USB drive is IN the computer. There are cables IN the computer that allow you to access the file that is ON the computer.
The same thing goes for TV. You would say "That actress is on the TV." but you wouldn't say in it, because she's not really inside your TV, she on the screen.
Typically, when referring to a file or program, you say that it is on the computer.
Which computer did you save that file on?I saved it on one of the computers on the right side of the lab.
For completeness' sake, 'in' is used if you are referring to a location of an object. For example, there is a hard drive in your computer. There may also be fans, a motherboard, cpu, etc in your computer.
Additionally, if you are referring to a specific location of a file on the computer, you might say:
Report.docx is saved in the 'Reports' folder. The Reports folder is in the 'C:\Users\Public\Public Documents\Work' directory.
If I were to use "in" that would connotate a physical location. I am in the house. The cat is in her bed. Being that data stored in a computer does not have a physical representation it seems strange to refer to it in this manner.
Contrast this with a filing cabinet where it makes perfect sense. The bills are in the file. (As another example, you wouldn't you refer to "music in a CD".)
When something doesn't have a physical representation I think you can safely refer to it as "on" the device (now here's where others post many examples where this doesn't hold true).