Disk image couldn't be opened. Resource busy.

[mattone]

Hi.
Using SD, I made a clone of an external hard drive (startup disk clone) to a sparse disk image on another external drive. I let it run overnight since it wasn't done when I was going to bed (~600GB). This morning I check the spare image to see if I can open it. Double-clicking brings up a small warning modal window, "The following disk images couldn't be opened." Below it lists my new sparse image with the reason, "Resource Busy." [Okay] (button).

I checked the logs but I can't find anything about the backup I made last night.

Mojave 10.14.6
Both USB disks are HFS+ journaled
SuperDuper 3.3.1 v121

Please advise.

Thank you,
Matt