Can't use My Passport 1Tb for mac

Hi! Just until two days ago my passport has worked perfectly. But then I just couldn’t find it… Not in my macbook pro, or my brothers mac mini. I could find it but not use it in my sisters PC, so i looked for updates and installed them. Now I find it and can read files, but can’t write anything on the harddrive. Anyone got any ides? 

Hi, try selecting the option to repair disk from the disk utility window.

Thanks, but it didn’t work… Any ideas?

It’s most likely corruption, if the files can be seen, but not written to.

Or, possibly, if you’re not safely removing the drive, but just pulling the cable, you could be locking the drive to being written to.  At least, that’s what I’ve heard.  Try connecting, then safely removing, to see if the drive gets unlocked for writing to.  If it doesn’t you’re stuck with reformatting the drive and starting over.  However, you lose the data, if you have to do that.