I think you may simply need to reset the backups for the PCs that are not able to backup to the Shared external drive that is attached to the Hub.
Is it configured as a Centralized backup for all PCs or is it a Network Share that you were using prior to 2.0 in the same arrangement?
Also, is the backup drive Shared with permissions set for All Network users allowed to have full control?
To reset the backup for the two other PCs -
Open OneCare
Click Change Settings
backup tab
Click change settings in the lower left
Make sure that the first screen for Centralized Backup has "No Centralized backup" selected
Click your PC on the left.
Uncheck the selection for "Turn on backup for this PC."
Click Next and save the revised plan.
Go back to the backup tab and click Change Settings in the lower left once again.
Select your PC and click.
Click Turn on backup
Select the change location link and select the drive you want to use for backup
Modify the schedule as desired for "When"
Select the file types as desired for "What"
Click next and save the revised plan with the new selection.
-steve