Changing the letter assignment for the drives worked. My workaround is to manually assign J to each backup drive.
When I plugged in the I: drive, I turned off the Symantec System Restore service and stopped the VProTray.exe process. I was still not able to eject the I: drive safely. I checked the handles and there had been three "System" processes with PID 4 that were open on the I: drive; after turning off SSR service and killing VProTray.exe, there were only two.
I had to stop sharing the I: drive admin share ($) in order to get the handles to close and the drive to eject.
After I changed the drive letter to J:, I was able to eject easily with no problems.
Symantec, can you recreate this problem? Small Business Server 2011 (which is built on Server 2008 R2).