Replacing late 2015 27" iMac flash drive

[Fishrrman]

OP:
"I did that, but it appears that if you have a completely dead internal drive it just won't work properly off an external drive. I can get it to boot and it can run some programs, but can't mount any hard drives or use any additional external hardware."

This is completely erroneous.
I've been booting/running all kinds of Macs (since my first Mac SE) since 1987 using external drives, both platter-based and SSDs.

Once booted, the Macs are completely functional (as if booted from an internal drive).

Before I bought this 2018 Mini (which also can boot fine from external drives), I booted and ran my 2012 Mini for SIX YEARS from an external SSD, its entire "working life". No problems.

Good to see that you got the iMac working with a replacement internal drive, but something wasn't right when you were booting from an external drive...

 

[coolerkid]

OP:
"I did that, but it appears that if you have a completely dead internal drive it just won't work properly off an external drive. I can get it to boot and it can run some programs, but can't mount any hard drives or use any additional external hardware."

This is completely erroneous.
I've been booting/running all kinds of Macs (since my first Mac SE) since 1987 using external drives, both platter-based and SSDs.

Once booted, the Macs are completely functional (as if booted from an internal drive).

Before I bought this 2018 Mini (which also can boot fine from external drives), I booted and ran my 2012 Mini for SIX YEARS from an external SSD, its entire "working life". No problems.

Good to see that you got the iMac working with a replacement internal drive, but something wasn't right when you were booting from an external drive...

But was the internal drive dead and still connected? I've always booted off external drives without an issue, this is the first time I was trying to do it with a dead hard drive inside and it was causing issues. Now with a working hard drive installed, I can boot off that same external drive I used before without a problem.

 

[pbedrosi]

Did you have to buy the OWC SSD / used Apple drive? or standard NVME drive + Adapter? I did the latter on my 2012 iMac 27" and it worked out well enough (still having fan speed issues to resolve). I'm just not sure if Late 2015 needs are similar (adapter).