I get to hear this very commonly, this flash drive sucks, windows cant even read it. More commonly once you insert a drive into Windows from a mac it asks to format the drive. Go ahead and format the drive and the mac no longer recognizes it as a storage media anymore. In all, life sucks!!!
Well there’s help at hand and indeed it’s quite simple. In it’s clean state you need to format a flash drive in a mac using the ex-fat file format. The walkthrough is pretty simple.

Goto Disk Utilities>Erase>Ex-Fat

And that’s all that is there to make a flash drive discoverable in both a mac and windows. Happy data transferring guys!!