Hello jandcsmom,
Thank you for using Apple Support Communities.
It is my understanding that your USB flash drive is not being recognized by your iMac. Since you hvae already reset the SMC and PRAM, I have 2 suggestions for you.
One is to test the issue while in safe mode as instructed in this article:
Try safe mode if your Mac doesn‘t finish starting up
Another is to run an Apple Diagnostic on you iMac to see if it detects any hardware issues. This article has the steps:
Using Apple Diagnostics
Cheers