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It depends on a couple other specs you can take a look at here:
That's a decent size. I wonder what it's TBW is? I separated it before (so it's not "fused" anymore) but now it's not detected by the system. I remember doing a Windows (Boot Camp) diagnostics to check with a Windows app, to see if things are good and left with it's read and write and left it for 8 hours or overnight. I wonder if it's TBW is all used up? Any recommendations how I can revive it? Thank you.
If memory serves, it was originally 128GB of solid state storage (as introduced by Phil Schiller for the first iMac of that form factor in 2012), but was later reduced to… something like 24GB in some subsequent iMacs?
Yep. And a correction: It looks like the changeover to 24 GB was in the 2015 model. I thought it was later than that 
Sorry I misunderstood the question, so that specs link from Apple didn't help much. But maybe this is better and aligns with what others here have said, from everymac.com,
"This model is equipped by default with a 2 TB "Fusion" Drive, which combines a 128 GB SSD and a 2 TB hard drive. It has a Serial ATA (6 Gb/s) connector for a 3.5" hard drive and a PCIe connector (PCIe 2.0 x4 NVMexpress interface) for the SSD."
everymac.com
"This model is equipped by default with a 2 TB "Fusion" Drive, which combines a 128 GB SSD and a 2 TB hard drive. It has a Serial ATA (6 Gb/s) connector for a 3.5" hard drive and a PCIe connector (PCIe 2.0 x4 NVMexpress interface) for the SSD."
iMac "Core i5" 3.3 27" (5K, Late 2015) Specs (Retina 5K, Late 2015, MK482LL/A, iMac17,1, A1419, 2834): EveryMac.com
Technical specifications for the iMac "Core i5" 3.3 27" (5K, Late 2015). Dates sold, processor type, memory info, hard drive details, price and more.
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2tb/3tb fusion drive Macs all got 128gb SSD portions.
The 1tb fusion drive Macs started out with a 128gb SSD portion, but that was "shrunk down" in later models to 24gb or 32gb.
OP:
You have been back-and-forth with numerous posts several times now, about presumably the same iMac that keeps giving you trouble.
What, exactly, is "the problem"?
Why don't you just erase both drives, and start over?
It's -possible- that one of the drives may be dead. If it's the SSD, be aware that an SSD can just "go dark" on you, never to be seen [alive] again.
If you can't boot from the INTERNAL setup, then you can boot from an EXTERNAL drive and get going again that way.
The 1tb fusion drive Macs started out with a 128gb SSD portion, but that was "shrunk down" in later models to 24gb or 32gb.
OP:
You have been back-and-forth with numerous posts several times now, about presumably the same iMac that keeps giving you trouble.
What, exactly, is "the problem"?
Why don't you just erase both drives, and start over?
It's -possible- that one of the drives may be dead. If it's the SSD, be aware that an SSD can just "go dark" on you, never to be seen [alive] again.
If you can't boot from the INTERNAL setup, then you can boot from an EXTERNAL drive and get going again that way.
FWIW, my 2014 2.8ghz i5 Mini has a 128gb SSD paired with a 1tb hard disk. I split them apart the day I got it and installed the system on the SSD. Have been booting from that ever since with no problems. It's just a media server and all the content is on a 4tb USB SSD, so 128gb is more than enough for a boot drive.
Thanks, 128GB is a significant capacity, I'm glad it's that big but at the same time it's sad coz' it won't get detected but then it can sort of detect but it doesn't know what it is. Windows too can detect is (Boot Camp) if you use the Disk Management app but it can't initialized it. If there're any ideas how to revive the 128GB (which I've "unfused" by accident last year when I was fresh installing things or maybe I used Terminal to unfuse it) Apple SSD with a sadly propriety connector (not M.2, Apple's version of M.2. Sometimes Apple does hit it when it uses a propriety something and it's better than Intel's innovations, 50/50 "home run"). I wonder what can revive that Apple SSD? Is there an app to know if it's TBW has been all used so I can move on and maybe just make that SSD a display on the wall (tech art) or something.Sorry I misunderstood the question, so that specs link from Apple didn't help much. But maybe this is better and aligns with what others here have said, from everymac.com,
"This model is equipped by default with a 2 TB "Fusion" Drive, which combines a 128 GB SSD and a 2 TB hard drive. It has a Serial ATA (6 Gb/s) connector for a 3.5" hard drive and a PCIe connector (PCIe 2.0 x4 NVMexpress interface) for the SSD."
iMac "Core i5" 3.3 27" (5K, Late 2015) Specs (Retina 5K, Late 2015, MK482LL/A, iMac17,1, A1419, 2834): EveryMac.com
Technical specifications for the iMac "Core i5" 3.3 27" (5K, Late 2015). Dates sold, processor type, memory info, hard drive details, price and more.
Thank you. God bless, Rev. 21:4
Hi. I wish I could have done that last year (after upgrading to Monterey) and do a fresh install with everything fresh including the freshly installed app and just erase everything and start from scratch but I can't erase the files and documents that are irreplaceable. As mentioned, I can't back up my stuff because it won't mount any external drive (even though on Windows, Boot Camp or on an actual PC it can). I can't even mount .dmg, .iso, .cdr and image files. Even if I fresh install the most stable macOS by far in Apple history (including the 90's) such as High Sierra (or the macOS before Big Sur which is buggy too, I think that before Big Sur was macOS Catalina) which I have can install on a partition or external drive, I won't have any files to get the files from a back-up coz' I've not been able to back up for a year now.
OP:
Time to seek out "live" help.
Either at a brick n mortar Apple Store, or perhaps a 3rd-party independent service provider who works on Macs.
If you have to "carry it in".... then... carry it in.
I have never ever (in 35 years) seen a Mac that couldn't connect and copy files to an external drive. Even when it wasn't working right.
Not ever.
Time to seek out "live" help.
Either at a brick n mortar Apple Store, or perhaps a 3rd-party independent service provider who works on Macs.
If you have to "carry it in".... then... carry it in.
I have never ever (in 35 years) seen a Mac that couldn't connect and copy files to an external drive. Even when it wasn't working right.
Not ever.
Hi. Yup, macOS Monterey's (I remember Big Sur having a problem with mounting too but rarely) the very buggy culprit, at least for older external drives and relatively older Macs. I'm tempted to just fresh install everything, start everything from scratch (all those decades of work- but letting go is peace sometimes
, forget the backup, a lot of the documents and files they're irreplaceabe but not life or death, then just go back to High Sierra (everything will work there for sure, like it's working now on Windows Bootcamp or on my PC backup), start anew, as if it was the first time using a Mac or computer again. I did buy four 2TB SSDs (MX500) just today, brand new for just US$18 each free shipping in that strange company called 'want.jp'(theirs names may be different in every platform in every country but it always has the 'want' and 'jp' coz' it's a japanese company with a unique business model, the items come from Shibuya, Japan- downside is takes weeks to ship, 3 weeks some say, at most) it's in every major online shopping platform covering all industries, including fashion and computers) that seems to threat the online shops like a casino or lottery to boost itself up (and it works, it's a top store worldwide).
The price is now back to normal, though at US$207 to $244- that was a huge blessing, divine providence level (it was by accident last night, I tried one last time for the lowest price as I was about to buy at this other shop). It was surreal, though it'll be considered clear once those four 2TB (total of 8TB SSDs SATA) are tested on Windows PC w/ SSD testing apps coz' it's extremely too good to be true. I just hope it's genuine and is really brand new, so it has the Crucial 5 year warranty (it can be refunded within 30 days but I'm deeply praying & hoping, that hassle will be avoided and it's really flawless, brand new and legit).
I'll open the iMac (I have the kit to stick the 5K screen back & an 1TB NVME bought last year w/ SSD stick adapter- there's that slight fear even today after a year of opening it up but courage is best, it's not like the old screen with magnets) try to back it up internal to internal but fresh install w/ High Sierra (Time Machine in Extended Journaled), once all files are in the 2TB, I used the other 2TB SSD external and back that up, then fresh install again High Sierra on the faster NVME and the 2TB internal SATA becomes the Windows Bootcamp disk (just games installed to be played on sundays, kind of rest after sunday worship/mass after all these complications)- it's a lot but w/ the extreme boost in bandwidth from a hard drive, it won't be so bad.
If Time Machine created by Monterey can't be restore to an older OS like High Sierra, I'll just copy paste the files manually (have done that before when Time Machine failed to restore- must be the layers of files of updates over more than 1 decade since Time Machine was launched) and reinstall the 3rd party apps that's compaitible w/ High Sierra at least.
Thanks for the reply and for the time, it's meditative to write when things aren't going well for a long time, if it's alright to share. Stay safe.