Old external HDD drive not recognized properly in new enclosure
Answer from bwDraco on SuperUser
This version posted as Community Wiki


  The enclosure exposes the drive to the computer as an Advanced Format 4Kn device, allowing the use of MBR for compatibility with
  
  Windows XP systems. When the drive is removed from the enclosure, the
  change in logical sector format results in an invalid partition table.
  
  
  External hard drives larger than 2 TiB in capacity often expose 4K sectors directly to the operating system. The 2 TiB limit commonly
  associated with MBR is tied to the traditional 512-byte sector size;
  4K sectors are eight times as large and therefore extend the MBR limit
  to 16 TiB. This allows the use of MBR on disks larger than 2 TiB in
  capacity, enabling use on Windows XP and other systems which do not
  support GPT.
  The underlying hard drive uses 512-byte sector emulation for compatibility with legacy systems. This means that while the disk
  physically has 4K sectors, the immediate host device (in this case,
  the enclosure) sees 512-byte sectors. However, to perform the
  aforementioned MBR expansion and enable Windows XP compatibility, the
  enclosure makes the drive appear to the computer as having 4K native
  sectors.
  When the drive is removed from the enclosure, the 512-byte logical sectors of the underlying 512e disk are exposed. This results in an
  invalid partition table that cannot be correctly interpreted. The
  746.52 GiB value you gave for the last "partition" when the drive is connected directly is precisely the amount of space that lies beyond
  the 2 TiB limit for MBR drives with 512-byte sectors.
  
  
  More details about the implications of this conversion are available
  in this blog post.