Cannot copy from MAC to Pendrive I attach pen-drive to my Macbook-air. It is detecting and I can also copy from my pen-drive to my Hard disk. But I cannot copy data from Hard disk to pen-drive. I tried with different pen-drives, but got the same result. 

What could be the reason for that? I have searched on web a lot but cannot know the reason.

Thanks in Advance !
The drive is probably NTFS, and OS X can't write to that. Either reformat it as FAT32/ExFAT using Disk Utility, or use an app like http://www.tuxera.com/products/tuxera-ntfs-for-mac/. 

The current top-rated answer is incorrect about OS X not being able to write to FAT32 partitions.