error about incorrect hardware installation

Got series of 30? 40? 50? stacked smallish error messages, stacked and staggered, referring to incomprehensible files (com?) OR, as I recall, "incorrect hardware installation." I cleared the messages, taking screen shots of several of them (should have used my phone), rebooted and could never get past a gray screen with white apple logo and spinning gear. I could boot to single user, but that didn't get me anywhere. Nor could I boot off regulation Snow Leopard disc.

Had hard drive replaced in 5-year old iMac because I need to continue to run Snow Leopard on one computer. Was told new drive is Apple-approved under a new setup that has a special cable -- I don't understand it, but it might be relevant to my question; as I got it, the heat/fan issue in iMacs limited drive options but now Apple allows newer/better/cooler? drives and what would have run the fan all the time is bypassed by a special cable. (Maybe this is all wrong.)

So after 2 days of faultless operation, having reinstalled most of my old stuff from a Time Machine backup (attempts to do a full disc image of old drive always failed after about 3-4 hours, so backup wasn't complete), I was restoring important software -- EUDORA! -- etc., then fixing sharing settings to bring some stuff over from my other operating Mac, when the series of error windows popped up.

Repair guys so far seem mystified, but I don't think they are listening to me, and they don't seem to recognize the error messages with the alternative "xxxxxxx.filename/setting/whatever" OR "incorrect hardware installation" language.

In 27 years of using Macs, starting with and SE through a fairly new Air, I've never seen this error, and I just had 30-50 of them.

Any ideas?