I want to make my memory card unreadable for my big brother and his friend's computers. Is it possible to make my memory cards unreadable only for computers but read and writable for mobile phones or anything like that to protect from a PC?
If you are using Windows 8 pro (or 7 enterprise), you can try Bitlocker To Go.
If not, you may want to try using Truecrypt - it isn't as straight forward, but, it will work very well.
If you want to know a more detailed guide on each let me know, but, there are many tutorials on this (and others) sites.