It is frequently experienced with external drives because people often reformat them before using them. Why? They ship with a whole lot of substandard bundled software which can take GB's of space.
When you reformat the drive with the Windows format and default settings, it will create a partition with a sector size of 4kB. The error with Windows Backup occurs because the System Image backup only supports a sector size of 512 bytes.
So the solution is to delete the partition and reformat with a sector size of 512 bytes.
First of all, you can check your drive's current settings with this command:
fsutil fsinfo ntfsinfo DRIVE_LETTER:
You will see something like this:
Bytes Per Sector : 4096 <<< this needs to be 512
Bytes Per Physical Sector : <Not Supported>
Bytes Per Cluster : 4096
Bytes Per FileRecord Segment : 1024
For Western Digital drives you can use their WD Quick Formatter utility.
For others, you may look for a similar utility on the manufacturers website, or a third party utility. I haven't tried any myself, but I did find PC Disk which may help.