preparing time machine for 15 hours?

I uses two external hard drives to backup my MBP 2011 running OS X 10.10.2 with 500GB hard drive by Time Machine.

I was traveling for 2 weeks with my MBP and ONE external hard drive with 500GB for backup and it was fine on my way. Upon returning home, I used the other external hard drive with 1TB to backup and it was running fine for one day. Then, However I mounted the 500GB hard drive to my MBP for backup purpose, it began "preparing time machine" for more than 12 hours until my MBP idle done. I restarted with 2 external drives mounting and this time it is the 500GB hard drive running "preparing time machine" for more than 15 hours.

I still let it run while monitoring "backupd" reading Bytes up to 71.27 GB and keep on reading....not sure whether I should keep on waiting or try something alternative?

Randy