Chances are your external hard drive is fighting for a drive letter with your HP printer that may have slots for SD, Compact Flash or other removable media types. Try unplugging ALL devices that have removable media and see if you can correctly install your hard drive.
If that works, assign your hard drive a high letter, like R:\ for "removable" then reinstall the other devices. You actually may be able to just plug them in and have windows re-detect them and all may work just fine.
To assign a drive letter to the drive:
right click my computer
choose manage
choose disk management
right click the external hard drive
choose change drive letter and paths.
If you do this and it works, you may have to reinstall any applications that are dependent on the paths that they were installed to. All your data will be intact, just the links in the registry will be off