Windows 7 Backup fails with I/O Error: 0x8078002A

Long story short, the reason this error occurs in general is because the partition is formatted with a sector size of 4kB. This is not supported in Windows 7, but is in Windows 8.
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.

References:

http://social.technet.microsoft.com/Forums/en/windowsbackup/thread/889b01be-d333-4fc7-b005-e12b7b236ad7

http://wdc.custhelp.com/app/answers/detail/a_id/6618/session/L3RpbWUvMTMxMzUzNzc5My9zaWQvT1hrMSpJQms%3D