Backup across multiple usb drives in single job

This is similar to viewtopic.php?f=18&t=27321 and multiple questions about backup to a group of external drives in viewforum.php?f=31.

I would really like a the option for doing a backup across multiple drives, but only 1 drive is connected at the time. It should of course support incremental backups.

Here is how I see work (fine tuning required):
1) User connects the drive and presses the backup button
2) Calculate free space on the USB drive
3) Job starts by creating a new file (master list) with all folder and file sizes the selected share+folder contains. The file is stored on the NAS.
4) In the root destination folder is the master list file copied over
5) Using the known size of the USB drive and an internal backup transfer byte counter, folders and files are copied using the master list until the next file will exceed the drive size. This step involves the actions of creating, deleting and checked for updated items.
6) Next file will exceed size, eject the drive and begin blinking the backup button (optional is notice by LCD and/or send email)
7) User removes old drive, then reconnects the next drive and presses the blinking backup button once. Maybe there should be a(n optional) timeout if user press the button within X time.
8) This is where loop begins/ends and go back to step 1
When job finishes the master list on the NAS can be deleted, or simply left there to be overridden at next run. Leaving it means the user can see what was (attempted to be) copied while the backup drives are stored safely - hopefully off-site.

A master list consists of:
path with filename, byte size, mtime

I know my steps are not complete and many details are missing, but the key is using a predetermined list. I have not addresses issues like what happens if a file is created/delete after the master list has been made, while job is running. But as long such issues are listed as limitations when creating the job, then I see no problems with it.