Hey there,
Etcher displays a validation error if it can’t ensure that every single byte was written correctly (it does so by using a CRC32 checksum).
Regarding the “broken” drive, Ubuntu ISOs are a bit peculiar, since they make non-standard modifications to partition tables to make the drives boot in some strange types of computers (see https://github.com/resin-io/etcher/blob/master/docs/USER-DOCUMENTATION.md#flashing-ubuntu-isos), and thus confuses certain operating systems.
Notice that the size that your operating system will display is the size of what’s defined in the partition table, which might be (and will be the case for Ubuntu ISOs) ignoring a part of the drive.
What you need to do is recreate the partition table, and then you can create partitions that make use of your complete drive.
On macOS, I believe you can wipe out the partition table by running diskutil eraseDisk free UNTITLED /dev/diskN on the terminal, where diskN corresponds to your drive (be sure to double check you don’t remove the partition table from another important drive!)