What is a file system which has r/w on all major OSes [duplicate]

"FAT32 would probably be your best bet, compatibility-wise. The only restriction is the ~4GB file size limit. With that aside, NTFS would be the best option if you planned on using file sizes > ~4GB, as r/w support is easily achievable on each of these operating systems and removes the low file size limitation.

With Windows, you're already in the clear
With Linux, most newer distributions come with the ntfs-3g driver as it is included in kernels 2.6 and above
With Mac, you can install ntfs-3g"