How do I move from one external drive to another, without the "Relocate Masters" command?

I have my masters referenced on an external hard drive, and I've been mirroring that drive with another so that I have a backup (the volume names were ApertureLib1 for the primary drive and Photo7 for the backup). I've filled that drive, so it will no longer be my primary master drive (i.e. all new masters going forward are going on a new, separate drive). I want to keep one of the two drives online and put the other away as an offsite backup.

The catch is that the primary drive (ApertureLib1) does not place nicely with my Lexar Firewire 400 CF reader; when I insert or eject a card, it has a habit of disconnecting itself. The backup drive (Photo7) does not exhibit this issue. So I decided that I'd just rename Photo7 to ApertureLib1 and be on my way; the folder containing all the masters (PhotoLib) is mirrored by using Arrsync (an rsync GUI), so all the files should be there.

Great theory, except that Aperture does not recognize the renamed Photo7 drive as ApertureLib1, despite the system seeing it as such (both in the Finder sidebar and showing up in /Volumes as "ApertureLib1"). The "locate referenced masters" dialog shows ApertureLib1 as 'offline' and does not give me any opportunity to point to the second drive as the master location.

I assume that Aperture is somehow tracking a separate unique identifier for the drive. Does anyone know what this might be and how I might convince it to treat the was-a-backup drive as being the One, True Location for this set of masters?

Many thanks.

(and I really don't want to use "relocate masters" because I'd first need to delete the exsiting copy on the second drive and then move over all of the images from the first drive, before copying them back to have a backup)