Unsafe Device Removal when creating SafePoint

I’ve been trying to create a new safepoint on my MyBook USB drive, and I haven’t had any success so far.  The same thing has happeend two days in a row.  The process begins and runs for several hours, but when I check its progress in the morning, it’s failed with these two error messages:

Unsafe device removal
A USB device (vendor: Western Digital, model: My Book 1130, serial number: 574D415A4132393335333733) was removed without being ejected possibly affecting data on share WD_MyBook_2.
Sunday, October 11, 2015 4:51:09 AM Code 1123

There was an error creating your safepoint.
2 - There was an error creating your safepoint SafePoint1 on the WD_MyBook_2 share on WDMyCloud.
Sunday, October 11, 2015 4:50:53 AM Code 1100

What’s going on here, and how do I fix it?  My firmware version is v04.04.00-308.