How to connect Galaxy S2 m250s model via USB? I have a Samsung Galaxy S2 that my uncle brought for me from Korea. This model of the S2, for some reason, has connecting issues through USB. The computer does not detect it at all!
I tried connecting it in my Ubuntu OS, and also on a cousin's Windows 8, still no luck at all.

I wish to backup the phone first, it is not rooted, so I need to follow what was said here. And after the backup I have to change its ROM to i9100 from the current m250s, so I can use it properly.

But for all that to happen, it needs be connected to the PC first.
It has ICS 4.0.4.

Additional information collected from the comments:

This is what I get when the phone is not connected:

$ sudo lsusb
Bus 001 Device 001: ID 1d6b:0002 Linux Foundation 2.0 root hub
Bus 002 Device 001: ID 1d6b:0001 Linux Foundation 1.1 root hub
Bus 003 Device 001: ID 1d6b:0001 Linux Foundation 1.1 root hub
Bus 004 Device 001: ID 1d6b:0001 Linux Foundation 1.1 root hub
Bus 005 Device 001: ID 1d6b:0001 Linux Foundation 1.1 root hub


And this is what I get after connecting through the download mode of the phone:

$ sudo lsusb
Bus 001 Device 001: ID 1d6b:0002 Linux Foundation 2.0 root hub
Bus 002 Device 001: ID 1d6b:0001 Linux Foundation 1.1 root hub
Bus 003 Device 001: ID 1d6b:0001 Linux Foundation 1.1 root hub
Bus 004 Device 001: ID 1d6b:0001 Linux Foundation 1.1 root hub
Bus 005 Device 001: ID 1d6b:0001 Linux Foundation 1.1 root hub


Which is no change at all. The phone is not even detected there! I also tried different cables, but the situation stays unchanged.
As you refer to the article utilizing adb backup, you want to use ADB to access your device. So for Windows, you would need to install the corresponding drivers; on Linux, a single line in /etc/udev/rules/50-android.rules will be sufficient.

I'd suggest you read my answer on the question What is ADB..., which (at its end) mentions a few guides to get you started with ADB. For a quick configuration on Linux, follow the steps in my answer on Configuring ADB for Nexus 4 on Ubuntu.

If on the Linux host the device is not turning up in the listing of sudo lsusb, and Windows does not even claim to have found an "unknown device" (as your comments suggest), make sure to rule out the USB cable as a possible troublemaker (try different USB cables; you might even visit a shop to "try before buy", best taking a Laptop with you). In case none of multiple USB cables work, a damaged USB port is becoming the likely cause -- though it might be charging fine, one (or more) of the data pins (or their connections to the board) could be faulty. Which would mean you'd have to turn in your device for service.