mac os and windows problem compatibility

[hitokiri1111]

i just bought a seagate external hdd (250gb) bout a month back and was working perfectly under the fat32 format tho i was told to reformat to the mac and get a mac os reader on my windows but i ignored it -.- silly me now for sum reason my hdd will not read on either computer and the hard drive sounds like it is trying to read from the hard drive but cant.... ive done this for a while wit my psp as a messenger between my mac and my pc but that worked fine so i dont know whats wrong with my external hard drive.... but the light button thingy that u press to tunr it on and off will flash on and off when it should stay constantly on...
the model number is: st30000U2

this one a hard one i tried to reformat to the mac os but now it doesnt read at all in my mac... on my pc is makes the hardware added noise but it doesnt show up on my computer at all... so yeah im stuck on this plz help if ya know how