Made a mistake by upgrading my DS710+ to DSM6, the internal USB (original) got corrupted.
Would like to restore this to orignal, and start experimenting afterwards, ofcourse with a seperate USB drive like you did (....and I should have done 🥴).
Are you willing to share an image of the original USB? It seems first partition looks ok, it asks to install DSM2.2-1042, it looks like this installs, but it never reboots again.
Problem seems to be in the second (99 mb) partition.
And like HellG mentioned, hardware should not be the problem, it looks like the software is checking or veryfing something on the USB bus. Interesting !
Thijs