Q: iMac Running El Capitan Locks Up When Saving Photos Library and the Mysterious "com.apple.sbd" Process

This issue is in reference to my iMac (27", mid-2011) running OS X 10.11 (El Capitan), 2.7 GHz Intel Core i5, 4 GB 1333 MHz DDR3, AMD Radeon HD 6770M 512 MBand, and a 1 TB hard drive.  The hard drive is about 2/3 full, but I can free up some space once I resolve this issue as discussed below.

I back up my Photo library routinely to my external HDDs (2 Seagate 5TB drives).  The library is 408.65 GB, and up through Yosemite, backing up the  library took about a day or so without issue.  Now with El Capitan, the iMac is locking up every time I try to back up the library, freezing up at approximately 1.26 GB (about 10-15 minutes) into the process.  The iMac doesn't lock up when I back up the much smaller iTunes library or individual files. 

I noticed in the Activity Monitor that the "com.apple.sbd" process is taking between 99% and 103% of the CPU and that it has been running since I last rebooted the machine.  One post I saw recommended disabling the key chain in iCloud to deal with the "com.apple.sbd" process, but that action made no difference.  Other processes (e.g., Outlook, "kernel_task", mds, etc.) intermittently spike into the 10-15% range, but for consistent offenders, "DesktopServicesHelper" or Activity monitor usually take the #2 place at approximately 7.0%

A few questions pop to mind:

1) What is the "com.apple.sbd" process?

2) Does the "com.apple.sbd" process need to run?

3) Is the "com.apple.sbd" process related to the iMac locking up?

4) Should I force quit "com.apple.sbd" process?

5) What else should I be looking at to see how to speed up the iMac?

A lot of the posts I've seen on the topic are so bland as to be unhelpful or represent actions I've already taken or am avoiding due to cost (like upgrading memory).

I keep the software up-to-date, but did I jump to El Capitan too soon?

It looks like OS X v10.5 and earlier and Disk Utility, but I can't find it in El Capitan.

I have nothing running in "Login Items".

I'm open to any ideas.  I'm sure there is something obscure that is escaping me.

Thanks in advance for your help!