Issues with time capsule back ups when external hard drive is connected/not connected

Question

I have a 2TB Airport Time Capsule (ATC) (tall tower type) purchased a few weeks ago and initially wanted it to back up a Seagate FreeAgent 500Gb external drive (formatted extended journalled) as well as three Macbook Pros.  The Seagate has a single partition called Lib and is not used for Timemachine backup; it just contains my iTunes library and lots of scanned and archived documents.  I want the Lib folder to be accessible to all three Mac users in the house and I've set up all of the Macs to connect and back up to the ATT.  They can all see the 'Lib' folder when it is connected to the ATC.
After a bit of searching on the discussion boards I realise that the ATC will not back up 'Lib' when the Seagate is connected by USB directly and that ATT will back it up if it is connect to my Mac and set not to exclude Lib  BUT I am encountering some problems with this scenario when I activate time machine. 
When I disconnect the Seagate by using 'disconnect' in the ATC viewer in Finder I shut down the drive before disconnecting the usb and reconnecting to the Macbook Pro just as a precaution.  When i connect and then activate back up using Time Machine selecting the ATC data folder, it is showing no previous back ups and never backed up.  When I had the Seagate connected to the ATT I was seeing back ups as far as 6th May when I bought the ATC.  I know that the Time Machine backups are not on the Seagate as I had to reformat it a few days ago and use Data Rescue 3 when I lost the ability to see any files on it after disconnecting it from the ATC.    I saw the 6th May backups in Time Machine before I disconnected the Seagate a few minutes ago so they must be on the ATC - just not showing after the Seagate was disconnected.  
Can anyone talk me through what may be happening here and advise on the best way of managing this.  I'm already planning to buy another external drive as the Seagate is getting a bit old now and I realise I should have secondary back up.  I should just say that the reason for bothering with the shared external drive is so that we can all access the photos and household files over the wireless network using ATT.  I would just like it to be backed up with minimal fuss if possible.
Many thanks in anticipation of some help with this
I

Answer 1

I would fix the problem by simply installing a more flexible backup software. Carbon Copy Cloner costs US40. And it is licensed so you can load it on all the computers in your house.
It can backup the external drive while connected to the TC, although if you are using wireless it will be sure slow.
Including the external drive in your backup with TM, is just messy. It will work.. and although you are not seeing the previous backups, they are there.. but TM cannot see them until it reconnects with the external drive in place.  Once it reconnects it will need to deep scan the previous backup.. you are going to find a simple backup takes far longer than you want. It is also difficult to recover from.
See Q5 here http://pondini.org/TM/FAQ.html
And several other comments in Pondini's KB will be helpful to understand. Sadly the guy who wrote it died before Mavericks came on the scene.. but it has more bugs than ever. TM has become less reliable for a lot of people.. and that is not doing any swapping of disks.
TM is honestly great for simple backups of your OS and particularly for incrementals but getting a clone of the disk like your external is far better for your setup.
See http://pondini.org/TM/Clones.html