Did you ever have any version of OS9 running on it?
For example, my Sonnet upgrade, which works without issue in OSX, requires a specific Sonnet-written replacement extension to run OS9. Your NewerTech may have similar requirements.
Do you have the original manual & stuff?
From the NewerTech 7448 installation manual:
"• Make sure the Mac OS version is 9.2.x (or higher) with the NVRAM OS 9
compatibility enabled. If the NVRAM patch for OS 9 was not applied, the Mac
will freeze at a gray screen when trying to load Mac OS 9."
There may be other reason(s) as well.