After copying the last version of Stretch distro on a 4GB USB drive with sudo dd bs=4M if=debian-testing-amd64-netinst.iso of=/dev/sdb1 && sync on a Lubuntu 16.04 system, the file system of sdb1 shows as 'unknown'. This is the case after creating a ms-dos or gpt partition table, a single partition, and formatting it with ext4 or FAT32 file system, which shows correctly after formatting, but goes to unknown after the dd command completes. Actually, the drive does not boot the computer, it simply gets stuck (although this might be caused by other reasons). I've also tried, with the same result, after executing isohybrid on the ISO file.
Unknown file system after copying Debian ISO with dd
filesystems;dd;iso
You need to overwrite the USB key including its partition table:sudo dd bs=4M if=debian-testing-amd64-netinst.iso of=/dev/sdb(sdb instead of sdb1). The download contains its own partition table.See the official instructions, which suggestsudo cp debian-testing-amd64-netinst.iso /dev/sdbsyncinstead.As pointed out by Mioriin, you should verify the downloaded image before copying it to your key, to make sure it was downloaded correctly and more importantly that it hasnt been tampered with.