External hard drive disconnected during reformat: now will not format
You could try overwriting the partition table with zeros. To do so, enter sudo dd if=/dev/zero of=/dev/disk3 bs=1m count=1 in a Terminal window. This will overwrite the first megabyte of the drive with zeros, which will include (and thus erase) the partition table (or its remnants). Remove and re-insert the drive; if everything goes well it will tell you that the disk was not readable and it'll offer to initialise the disk. (I tried this with an old USB stick a minute ago and it worked fine.)