Hi Francis,
This could be due to the effect of transitioning the physical format of hard disk drives from 512-byte sectors to 4,096-byte sectors.
What is the drive type that you are having on the hard drive which you are using as a backup drive?
I would like to inform you that the data storage industry such as Seagate have configured their large data drive to use 4096 logical sectors (also known as 4K or 4KB sectors) from 512-byte type. This transition is driven
by several factors including increase in storage density and reliability. Although, this transition causes incompatibility issues with existing software (including operating systems and applications).
You can refer to the link to get the details on this update:
“Microsoft support policy for 4K sector hard drives in Windows”
Refer to the information provided for "Windows 7"
http://support.microsoft.com/kb/2510009/en-us
To check for the kind of drive that you are using current with the hard drive, you can also follow the steps provided on the link.
For further assistance on this issue and the updates related to it, you can contact Seagate. Refer to the link to do so:
http://www.seagate.com/about/contact-us/technical-support/
Please feel free to ask if you have any other query on Windows.