Hard drive compatibilty b/w Mac and PC

[macuser19]

Hi, I connected my Western Digital My Book hard drive to my Mac by mistake (it's the one I use for my PC). The lights started flashing up and down. When I reconnected the drive to my PC, the PC no longer recognizes the hard drive. What's happened? Thanks

 

[Hellhammer]

Open Disk Utility in Mac and see what is the format of the drive (should be NTFS or FAT32 so WIndows can use it)