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.