This will help you, I guess.
After the first attempt with NTFS filesystem, I had upgraded my firmware to Security Patch Firmware (1.06.B03). Now, to test once more the HD, I formatted it to FAT32 again, but still I wasn't able to get it working.
So I downgraded to 1.04.B12 and now it's correctly reading the FAT32 formatted HD. So it seems that this new firmware has some problems mounting USB devices.
But still, I formatted again with GPT + ext3 and it doesn't mount it. What can I do?