Ok thank you:). You're right I have tried connecting with other Xbox 360 as well and the same happens. My Internal HDD its also a 500GB and is full :( sadly i'm not sure if i can get bigger one. :( But only thing i don't get if it is corrupted, the same external HDD is still working totally...
I having the same issue as well! My external hard drive was working fine 2-3 days ago. Now when i connected it to my xbox yesterday it shown as "Unformatted" inside Storage Devices. And XEX Menu and FSD not detect it either. And it is Formatted in FAT32 its a HGST Touro 500GB.