This could be several issues
is the Sierra version from the apple website recently?
i have every OSX on a ssd drive that does the same thing sometimes.
ex: MBP 2012 usc of Catalina wanted the latest dated download,
and stopped during an usb a 3.0 upgrade for me last December.
(im baffled over this as well)
if this is fresh from the apple download page, you might need to upgrade the firmware
of the older OSX as in El Cap, Yosemite, Mavericks etc might not have the proper data to go to Sierra.
The sierra usb drive could be not up to par or missing some data, or just slow as 2.0 USB drives get sluggish
i would update the latest OSX on the current mac then upgrade via the app store if possible.
there should be others here adding their input soon, perhaps they know something that I overlooked.