Thanks for your response, Dan. "Yes! This is an issue with in Apples newer MacOS EFI/SMC updates." When I posted the above, I thought it was a problem with my MacBook Pro; I've since learned that it's a common problem afflicting hundreds (at least) of Mac users. It is beyond ironic that the software Apple offers to its users to diagnose problems with their Apple hardware… doesn't work. So we're all waiting for Apple to get off their collective posterior and fix it. "There's a ton of people encountering this Unable to launch Apple Diagnostics Apple altered the EFI so all versions of MacOS are effected from Sierra on up is where I've seen it." Yes, I also posted my query at Apple Discussions: MacBook Pro 2013 - cannot load Apple Hardware Test, and was referred to the Unable to launch Apple Diagnostics thread (which now has 163 "I have this problem too" clicks, while mine has 5). See my posts here and here for more detail and follow-ups. I spent an hour and a half on the phone with an Apple Senior Advisor on...
Well, I went ahead and tried with the Torx #6 driver, very carefully, and it worked; I got the four little set screws out and was able to swap the mounting flanges. Tip: Don't completely remove the screws; they're tiny and a bear to reinsert, and they have to be unscrewed only about 1mm to get the flanges out. So, after two years, got the MacBook all together again. And discovered that the battery is now swollen, so I have to get another. Anybody have any suggestions for a good but inexpensive battery? I don't really expect to use the MacBook as a portable anyway, so don't need a heavy duty battery, but it should have one in it.
Check this bracket to make sure it isn't bent. I had a MacBook that would intake or eject discs with difficulty; there seemed to be some obstruction in the disc drive opening. I took it apart and determined that the case opening (CD slot) was slightly out of alignment with the opening in the disc drive. I couldn't see why until I noticed that the front end (near the case opening) of this bracket was bent, so it didn't properly slide into the "slot" in the case chassis where it's supposed to go, thus forcing the disc drive to sit an mm or so high at that point, so it wasn't aligned with the disc insert slot in the case. I removed the bracket, straightened it, reinstalled it, and now everything works fine.
It should be noted for reassembly that this bracket should be slid toward the back end of the drive when it is inserted, then slid forward toward the front end so its "tab" goes into the "slot" in the chassis; then the bracket will be in the correct position to insert the screw. If the drive is put into its place with this bracket slid toward the front (opening) of the drive, there is danger the bracket may be bent, causing the problem I experienced. (I don't know when it happened in this unit, maybe when it was first assembled in the factory.)
I didn't look closely at these screws when I took it apart, but when I went to put it back together (two weeks later) I found I had two longer (4mm?) screws, and one short one. So I put the longer ones in the two end spots, the short one in the middle; they all seemed to go in okay. (Maybe the screws got changed around the last time the cracked top case was replaced, when the MB was in the Apple shop under warranty last year.)
The top two screws didn't seem to want to come out, and I saw no reason they had to, so I left them there, put a little piece of tape over each so they wouldn't fall out.
Check this bracket to make sure it isn't bent. I had a MacBook that would intake or eject discs with difficulty; there seemed to be some obstruction in the disc drive opening. I took it apart and determined that the case opening (CD slot) was slightly out of alignment with the opening in the disc drive. I couldn't see why until I noticed that the front end (near the case opening) of this bracket was bent, so it didn't properly slide into the "slot" in the case chassis where it's supposed to go, thus forcing the disc drive to sit an mm or so high at that point, so it wasn't aligned with the disc insert slot in the case. I removed the bracket, straightened it, reinstalled it, and now everything works fine.
It should be noted for reassembly that this bracket should be slid toward the back end of the drive when it is inserted, then slid forward toward the front end so its "tab" goes into the "slot" in the chassis; then the bracket will be in the correct position to insert the screw. If the drive is put into its place with this bracket slid toward the front (opening) of the drive, there is danger the bracket may be bent, causing the problem I experienced. (I don't know when it happened in this unit, maybe when it was first assembled in the factory.)
I didn't look closely at these screws when I took it apart, but when I went to put it back together (two weeks later) I found I had two longer (4mm?) screws, and one short one. So I put the longer ones in the two end spots, the short one in the middle; they all seemed to go in okay. (Maybe the screws got changed around the last time the cracked top case was replaced, when the MB was in the Apple shop under warranty last year.)
The top two screws didn't seem to want to come out, and I saw no reason they had to, so I left them there, put a little piece of tape over each so they wouldn't fall out.