Is it a Seagate ST2000DM001?
If it's a standard drive, I would be curious to see whether a standard 4 TB drive would work. In earlier versions of the Time Capsule there have been problems recognizing the full capacity of the 4 TB drive.
Also, in the newer iMacs Apple has been using harddrives with modified firmware, so it actually wasn't possible to replace the drive. Wonder if that's the case here with this 2013 Time Capsule.