Can't back up from network drive to portable disk?

In Backup & Recovery 16, running under Windows 10 64-bit.

I want to set up a job that will back up a folder residing on a NAS drive to a portable disk. The network drive is mapped to U:\ and is perfectly visible in Backup & Recovery 16. The local portable drive is mapped to D:\. When I attempt to define the job, I cannot Finish job setup -- I see 'Operation Failed'. Additional information: 'Object Manager 0x1000101'.

I can define jobs that back up from local hard drives (C:\, E:\) to the local portable drive (D:\) without any diffculty.

What am I doing incorrectly?