What's on the external hard drive now? If it's only the Windows installation data, then I'm assuming you wrote to the external hard drive instead of another USB drive (or you intended to do that...).
Anyway, as long as you've only overwritten 30GB out of 1024GB hard drive, then there should still be about 994GB of the old data still there.
Recovering at least some of the old data should be possible, a recovery program like TestDisk, it tries to do things like:
TestDisk is powerful free data recovery software! It was primarily designed to help recover lost partitions and/or make non-booting disks bootable again when these symptoms are caused by faulty software: certain types of viruses or human error (such as accidentally deleting a Partition Table). Partition table recovery using TestDisk is really easy.
There's good documentation at the link above, including a Step by Step guide.
Or if TestDisk fails, you can always read the whole drive directly, looking for files to piece back together with PhotoRec. Recovered files might not have their original names or be in the any directory trees, but at least you can get some of the data back.
Recovering the drive capacity should be just a re-partition (new GPT/MBR) & re-format away. But don't do that yet if you want to recover any data.
DiskMgmt.msc
). regarding the lost data, i have no out of the box offer for the trouble you had to resort on third party recovery tools.. :(