I’m having this issue intermittently on Catalina using 3.5.6 and 3.5.7b1. It usually occurs the first time, and then if I re-run DropDMG (using the command-line tool) it often succeeds:
[exec] Error: hdiutil: flatten: flattening "test.dmg"... [exec] hdiutil: flatten: "test.dmg" failed: Resource temporarily unavailable (-54) [exec] hdiutil: flatten failed - Invalid argument
I’m using the latest Catalina beta at this time. Is there anything more I can provide to help diagnose this issue?