For one once you transferred the image made off of the larger Seagate drive and placed it on the WD 160gb model that saw an immediate hardware change resulting in the badd_pool_header error. You would need a clean install to see that corrected.
The previous installation simply bumped the Seagate drive letter down having only added that to the previous configuration. If you were to perform a clean install of Windows on the 160gb you would likely see the Seagate as D until going into the Disk Management to reassign drive letters there.
Seagate should have some of their drive tools you can download off of their support site like drive diagnostics to see if there are bad sectors or other problems with the drive to report. That would be SeaTools found at Seagate Technology - SeaTools