Time Machine wants to make 2nd copy of ext drive

Hi,

I'm sorry if this issue has been discussed elsewhere. It seems like a simple thing, but it's difficult to briefly describe.

Firstly, here are the specs of the items involved:

LaCie RAID Rugged 4TB

Mac OS Extended (Journaled) [all 4TB, i.e. no partitions]

1.9TB used

LaCie SSD Rugged 256GB

Mac OS Extended (Journaled)

108GB used

MacBook Pro (Retina, 13-inch, Late 2013)

2.4 GHz Intel Core I5

8GB 1600 MHz DDR3

OS X Yosemite (10.10.3)

Mac OS Extended (Journaled, Encrypted)

256GB SSD

115GB used

2013 802.11ac AirPort Time Capsule 3TB

firmware version 7.7.3

Time Machine is able to perform a successful initial backup of the MBP and 2 ext drives to TC. With the SSD ext drive, Time Machine will continue to recognize it and back it up normally, even if it is not present for every backup.

With the 4TB ext drive, once the drive has "missed" a backup (like, if it's been ejected and unplugged during a backup), then, the next time it's plugged in during a backup, Time Machine wants to create a 2nd backup of it. Or at least that's my guess, because all Time Machine says is that there's not enough space to back it up, and the amount of additional space Time Machine says it needs is what it would need to back that drive up twice.

Time Machine gives me no more info, so all I can do is erase the TC drive and start over, which is a 32-hour process. I've done that a couple of times anyway, to see if the problem is recurring, and indeed it is recurring.

Does anyone have a clue as to why this might be happening? I also have an old iMac with multiple ext drives, and I've never had a problem with those, or with my MBP until I got this new RAID Rugged. So I figure it must have something to do with that, except I have the entire RAID Rugged drive formatted to Mac OS Extended (Journaled), just like all the other drives.

Thanks for any information that can help me solve this.