Ok, I managed to replace the starter successfully.
Thanks to all the wonderful folks here who gave me the right info on how to do it 
But now, I have another question.
Is there a way to find out what the problem was to see if it’s fixable?
It turned very slowly and with great effort, and at times even seemed to grind to a stop, but managed to turn again (very slowly) until the motor miraculously started.
If it’s fixable, I’ll keep it as a spare.
A bench test may turn up a fault in the starter or the starter solenoid.
Not knowing the past history on your starter/starting problem I can’t offer much in the way of help.
If it was engaging, I’d not personally consider the solenoid a likely candidate. Since it was grinding so slowly I’d bet on either shorted windings or toasted brushes.
Yes, could be too Mb, but if what he says happened with the action of this starter suggests anything, I’d begin with a low battery or bad ground connections or bad battery cables/connections, before replacing an expensive starter.
I didn’t continue with my post as I don’t know the past history here.
If the starter solenoid is mounted to the starter, the large contacts which the solenoid brings together and which transmit the battery power to the starter motor could have arced and pitted. Disassemble the starer solenoid, file and sand those contact blocks. Then, you could load-test the starter, someway (on a test bench or on a car).
There are bushings at either end of the armature that can fail. If one fails badly enough the armature can contact the field windings and “grind to a stop”. There are spring loaded carbon brushes that ride on the armature’s commutator, and these brushes can wear away until the starter current can’t transfer to the armature, again making the armature “grind to a stop”. The field windings or the armature windings can short, again causing the armature to “grind to a stop”.
You’ll have to dismantle the starter to find out what’s wrong with it. They’re pretty easy to dismantle and not difficult to repair. Take it apart; you can’t hurt it much, it’s already broken.
There are Alternator/Starter shops that can rebuild/repair your starter. I hardly ever buy a new one, just have mine rebuilt. It’s usually pretty reasonable. Depending on what’s wrong, I get alternators rebuilt for between $40 and $80 and starters somewhere in the same neighborhood.
Skipper
Roadrunner made an excellent point. Since you’ve apparently just changed the starter and the problem, the slow grinding start, hasn’t changed, it may be appropriate to now consider weak voltage as the problem. Perhaps a good check of all the connections and a good load test of the battery is in order.
Um, Mb, he didn’t say the slow starter was still a problem on the vehicle now that a another one was in it’s place.
He’s wondering what may be wrong with the one he took off. I think that’s what he meant.
Like I said, I don’t know the past history leading up to the starter change.
What I suggested was what I would have done prior to making a change.
UNless there actually IS something wrong with the starter or solenoid, perhaps the Cleaning of the battery connections may have rectified the problem.
If such is the case, then there wouldn’t be a need to replace the starter.
I always start with the simplest methods first. Not always successful though.