I've got a (old "flat" version) 1TB Apple Time Capsule and I've configured my Macbook Pro (running El Capitan) to backup data on the Time Capsule, through Time Machine.
From the top-right information on the menu bar, I can see that backup is working fine. Nevertheless, after backupping almost 2x the size of my hard drive in about 10 days (I'm not always attached to the network and I send to sleep my MBP during night), the first backup is not completed yet.
The bad part is that backup information shown in the menu bar display increasing full backup size: if currently remaining backup size is about 5GB, in an hour it is almost the same size, since it continuously increases. Therefore, since backup size increases just like data transfer speed, backup is never ending.
Is it an incompatibility of El Capitan with older Time Capsules? How can I solve it?