I had endless problems with Ghost 2003 and USB, and often found that the image integrity check at the very end failed - as you say hours after the start. Then a friend got Ghost 9.0 which (for XP) does not use PC DOS, it uses 'Microsoft .Net' (for other OS you have to install the Ghost 2003 disk). Officially this allows you to keep working whilst the image files are being created, but being a sceptic, I just let it get on with it and leave well alone. I have not yet had a failure, and do a 'ghost' at least once a week. (I have purchased my own software, by the way!)
Re-formatting the external drive probably is the best way to make a clean sweep and return it to factory condition. I'm not sure why the system had slowed to a crawl - that would need to be analysed at the time to see whether drives had filled up, temp folders, recycle bin etc.