I can install Windows 10, but not MacOS on my iMac (Paragon disk manager). HELP!

Hey Guys! Thanks for the advice. I did in fact reformat the drive a number of times as specified. I'll do it again later one more time, following your instructions to the letter.

Now, even if I attempt installs on external HD's or other SSD's the results are the same... The installers take forever to even get to actual tools/installer, then the install itself crawls, 10.8 being the least problematic (El Cap, Hi Sierra and catalina do not complete even when left over night).

I still suspect there is an issue with the EFI, but like I said I will follow the clear instructions you left here to the letter and cross my fingers...

Hey Guys! Thank you all for the input. I took a day away from thinking about this, as it was literally driving me nuts! Here's an update...

The usb installers used were used numerous times on other machines for successful speedy installs.

The SSD is a 2.5 Apple branded toshiba originally from a 2011 Macbook pro that I used successfully before this incident, EVEN in the machine in question (iMac) prior to the PARA "GONE" software install...

I opened the iMac up and removed the SSD and reformatted it on another machine using a USB case, as on the iMac it was failed in a few attempts to delete the final partition last time I tried, and generally is close to unusable with the stuttering and delays.. It was easily formatted in this manner.

I booted the iMac with NO internal SSD using the Catalina USB installer as well as an external SSD with a clean install of Sierra. Booting from both was slow and neither successfully reached the desktop. I only attempted this once and I plan on attempting other options today...

HWtech: Your proposal of trying the Knoppix drive sounds promising and I will attempt to follow your instructions this afternoon...

At this point I am still convinced PARAGON did something to the BIOS/EFI "whatever it may be called" :-) on the MB, but as always, I KNOW that I DO NOT KNOW enough to be sure :-) Since this is a clients machine, I plan on moving forward with replacing the main board tomorrow as he's been waiting almost two weeks and I just want this nightmare to be over!

In the meantime, I am keeping my fingers crossed that there is a solution not requiring this drastic measure and hope the additional info here helps greater minds than mine to suss out where the problem lies! As always, I am so grateful for everyones time and effort in helping me to decipher this strange magic that has cursed this poor little iMac!

PS: HWtech: I am a newb with Linux, any chance you could send a direct link to download 8.2 Knoppix? I am having zero success finding what I suspect should be an .ISO? This is the closest I've come, but i get the feeling none of these download links id what I need... ftp://ftp.uni-kl.de/pub/linux/knoppix-dvd/md5-old/

PSS: The clean install of Sierra made it to the desktop, but exhibits EXACTLY the same behavior as the SSD which is no longer attached to the system... I am going to try booting again without the Catalina Installer attached, just to be 100% sure it has nothing to do with it...

I used disk Utility "restore" function... Right now I have the screen off and put a brand new SSD in it and am attempting to load Mountain lion from the internet recovery option... Nothing but monitor and keyboard/mouse attached.

Sorry for not posting in a bit. I had to step away from it to get perspective... I replaced the MLB and am having the same issue. Yeah, I realize it was futile now! Arg! really?

Anyway, I'm going to try more suggestions here, but found something curious and interesting. When I tried to run diagnostics using the "D" key on startup, I get an error!

error 0x8000000000000003, cannot load 'EFI/Drivers/TestSupport.efi'

I get this message regardless of what drive "I've tried about 6 at this point" I have in it, or even no drive at all!

This is with the replacement MLB in the machine. I am so absolutely baffled at this point.

Anyway i decided to post this before following some of other advice still pending here, as something about this would seem to be related in some way to my main problem... So grateful again for all of the input and hoping beyond hope this toidbit sets off a lightbulb for one of you higher level techies than I!!!!

Attempting this now Loner T... I get the same error mentioned above with only the blade SSD installed. It seems exactly the same as the other drives... It starts booting super fast and looks good. then it gets about 2/3 way across the bar and slows to a crawl for the rest of boot. Gets to login screen and beachballs for bit, then logs in to jerky mouse movements and hit or miss click responses...

This is absolutely the most pernicious and annoying issue I have ever run into in my "wow!" almost 20 years working with Macs... Even replacing the entire MLB gives the same result... Loner T, any thoughts on this error with the EFI drivers and my initial suspicions regarding something screwed up with EFI being the root of this? I saw a couple of people other places suggest having the folks at the Apple store Flash the "EFI Firmware"...

I am at a total loss and my client is getting anxious that he may never have a working machine back in his hands. To be honest after such a dismal result from the MLB replacement, I am really worried myself that buying an entire new machine for him may be my punishment on this FUBAR...

Also, I am just baffled where this screwiness could be coming from? I would think the new MLB has it's own firmware, so if it was that, it should resolved the issue? I tried multiple keyboards and mice, multiple sets of known good RAM, multiple SSD's and HD's (ALL WORKING FINE IN OTHER MACHINES). Even now with nothing but a keyboard, mouse, monitor and the Blade SSD. Everything goes to **** 2/3 way through the boot? I noticed the fans go on at the same time as the lag starts FWIW.

I know the variety of SSD used in the blade slot is sata based as well, so something with the main I/O would be my strong suspicion. Something related top low level operations from the behavior of the mouse/keyboard...

Does anybody know what is happening during boot when the fans change speed about 2/3 of the way on the progress. bar?

Hey Loner T, I booted my 2012 MacBook Pro from every OS I tried on the iMac successfully and FAST. The Boot OS on the Blade was operating perfectly fine in the original early 2013 13" MacBook Pro Retina and then in a Late 2012 15" retina.

Its' funny one of the very rare times I am desperate enough to visit the apple store for help and they are closed up! :-)

I am worried they will offer no help in that respect because I've had the thing opened and have upgraded components... I just want this nightmare to be over for my client. He is the nicest guy and very understanding (he actually thought I was going to charge him for the new MLB!). I can tell he is getting worried now and I don't blame him. I've never felt so hopelessly out of my depth and it stinks!

Lone T, do you have any hope a firmware flash might resolve this and do you think they would be amenable to doing that for me when they reopen? Gosh, they have to re-open, right? :-/

:-) I am very familiar with that type of issue. There is no reason to suspect that here, I only mentioned it at all due to the timing of the fans changing speed around the same time the progress bar goes to turtle speed. They don't run full blast and there are no signs of overheating whatsoever...