Sata cable questions on iMac late 2013 21.5"

[Nessdufrat]

Hi!
Several questions here. First things first: I found this IMac in the trash. yes. Crazy. so it was kind of broken, the glass panel was shattered, the hard drive was missing, the power board was toast, the power button had been glued to the metal casing for whatever reason... this iMac had suffered. that is to say, I have no idea what happened to it before I got it.

Anyway, because it was waaaay better than the computer I had at home, I decided to take it and fix it. So I changed the power board, repaired the power button, and tadam, I managed to get it to start up. No image, of course, since the lcd panel was not in the best of shapes. Plugged on an external monitor, it was working fine, of course it had no hard drive so it couldn't really do anything, but at least it downloaded the recovery os.
I bought an SSD (Samsung 850 Pro. I have the 840 pro in another computer, I love it), I put it inside, and.. nothing. At first I thought maybe it's because it was not in the correct format. So I went ahead and actually cloned my system on it (el capitan). Nothing. I read somewhere that sometimes the Sata cable is good enough for a hard drive but not for a SSD. So I put a hard drive inside. Nothing. Obviously, something is wrong with either the Sata cable or the Sata connector on the motherboard.

Thing is, I went to the Apple Store a while back for something completly different and we talked about the "trash iMac", and at that time, about the lcd panel (I had just bought the SSD, and I hadn't tried it yet). The guy told me the problem had to be with the lcd panel, it couldn't be anything else (I suggested faulty lvds connector on the motherboard, I could see he was trying hard not to laugh at me). So I went ahead and bought a new lcd panel (second hand, tiny crack, fully working from a computer shop. Didn't pay a lot for it but still). Guess what? It was not the lcd panel.

So today, I went back to Apple with my trash iMac in a plastic bag, it was really something, lcd panel was taped to the casing, power button was hanging outside the casing, stuck to it with another piece of tape... They had their fun. They had told me they could run a free diagnostic on it, so I brought the computer.
Turns out, they couldn't tell me anything. So I still don't know whether the problem is with the Sata cable or with the Sata connector, if the video problem comes from the lvds connector or from the graphic card (one thing is sure by now, it's not the lcd panel, thanks a lot, Apple guy from last time, I could have spare 65 dollars), but I can't help thinking that's a lot of probkems for one computer.
A Sata cable is not that expensive, but changing it means taking out the whole motherboard. And I think it's rather strange that lvds connector doesn't seem to be working, Sata connector isn't working either.

Anyway, here's to my question, after this very looooong introduction. The guy I was talking to was super nice and as he could see I was genuinly interested, he showed me some schematics, and one in particular, which showed that there was a secondary Sata connector on the motherboard. I'm not talking about the legendary SSD pci port at the bottom of the motherboard, but a Sata port just behind the one which is currently in use. He told me there are two (actually three) different Sata cables for the iMac : one is the hybrid cable, that's the one I have in my computer, it delivers the power as well as Sata to the drive. One is a single Sata cable, one is a single power cable. On the schematics, we could locate the single Sata connector (its right behind the usual Sata connector, the one that "hosts" the hybrid Sata cable) but we weren't able to find where the power cable is supposed to go on the motherboard.
I've been doing some research for a while on this Sata cable matter (I had my appointment at the store today, but I've been looking at the problem for about 10 days already) and I never saw anything about this additional Sata port in this particular computer (late 2013 iMac 21.5", namely emc2638, a1418, if my memory is correct). He couldn't find anything else on his schematics and documentation either, and couldn't tell me anything more about the second Sata port, apart from the fact that it's not possible to have two hard drives inside the iMac (it's not designed to host two HDs), and that it probably wouldn't work as the two Sata ports probably are used together in "some cases".

Does anyone know anything about this secondary Sata port? In my situation, what would you do? Would you try to change the Sata cable (and risk messing something up with the motherboard) or would you simply drop the matter altogether, go buy yourself a thunderbolt enclosure and put your SSD inside?
In that case, which one? I do realize my 850 pro is very fast. Thunderbolt 1 wouldn't be fast enough (that's what the apple guy said. I guess I believe him, a friend of mine has a thunderbolt 1 station and did some write and read tests on his ssds, the speeds were not that awesome). I was looking at the startech thunderbolt enclosure, but that's thunderbolt 1. I see OWC has launched a thunderbolt 2 enclosure for 2 disks, that would be perfect, but of course, I live abroad, and with shipping and taxes, the price is insane. I'm considering selling my 850 pro (it's almost brand new) and getting a LaCie SSD thunderbolt. It wouldn't be as fast, far from it, but at least it wouldnt be underused.

Any advice on any of that, guys? I know I'm asking a lot, sorry, I probaby should have done a bunch of different threads...
Thanks!

Edit : seems like the guy at Apple didn't really know his products : late 2013 IMac doesn't feature thunderbolt 2, it was only introduced in late 2014 iMacs and more recent computers. So the OWC dock would be a waste on this mac.

 

[OldGuyTom]

It sounds to me like you know more about the computer than the "geniuses" do.

If it was me I'd put the SSD in a USB 3 housing and try to boot from that. I've had problems finding any reasonably priced Thunderbolt housings, but USB 3 are a dime a dozen, they're cheap, and they're fast. At least this way you could get the thing running for a low cost and see what all it's problems are.

good luck.

 

[Nessdufrat]

Oh, I did that already. I got it running on a clone of my system (el capitan) and it was working really well. I got a fan control application to get my fan to stop acting crazy, and the computer was totally usable (on an external display) But the problem is that trim can't be enabled in a usb 3 enclosure with my SSD. And my SSD is the fastest one on the market, it's a shame, usb 3 seriously limits the write and read speeds on it. If I had gotten a Crucial SSD, I would have chosen the usb 3 solution, but with the Samsung 850 pro, it's a waste.
I actually ended up buying a LaCie SSD thunderbolt second hand from somebody, I'm getting it next week, probably. I'll be selling the Samsung SSD, since I have no more use for it.
A guy I'm talking to on a French forum has the same computer and he has the version with the SSD (I don't, mine is the one with the hard drive only). He had to take his out because it got struck by lightning (not the computer, obviously, but the surge fried it) and he's considering replacing it. So he took the opportunity to take some pictures. Turns out, it's the same SSD that's used in the MacBook Air 2013 (too bad, I have the 2012, and I'm upgrading my SSD this week... It would have been so cool to try it with my old SSD...). I found one on eBay in France, if it doesn't get too pricy, I might buy it to try it in my iMac. In that case, I would buy a Sata cable and change mine, since I would be taking the motherboard out anyway.
I must admit, I'm really curious about this secondary Sata port. Also, I'm curious about this SSD port on my iMac. Ifixit seems to say it's not soldered and it might be possible to put a SSD inside, but even though some people commented on that saying they wanted to try it, I couldn't find anybody would did report trying. The guy from the Genius Bar told me it was a possibility, although he thought it was getting complicated and expensive (basically, he told me "try the Sata cable if you can find a cheap one, just be careful when you take the motherboard out, and if that doesn't fix the problem, drop it and buy a thunderbolt enclosure to put your SSD inside").
If I buy the SSD and try it on my computer, I'll post pictures of the motherboard, so that the matter will finally be solved ^^

My biggest disappointment is the lcd panel... But I have absolutely no idea how to fix that. I know my panel is working, I bought a new lvds cable when I was doing some tests with my old panel, so it's not the cable. Maybe it's a tiny something, like a fuse, but I wouldn't even know where to begin.

 

[Nessdufrat]

So, I disassembled my iMac with the intention to put a SSD inside (ordered, not received yet, but I wanted to go ahead with the work). The Sata port mystery is solved. It's not a secondary Sata port. It is THE Sata port. The second part of it, actually. Once the iMac was open and the motherboard was out, I discovered my Sata cable was partially disconnected from it : the big plug was still in (data, I guess), but the small plug was floating around loose (power, I think). Anyway, looking at the Sata cable, both are needed to make the drive work.
I plugged both back in. I'll find out what happens when I reassemble everything, hoping I didn't mess anything up in the process.

Sata cable : the small plug was disconnected.

Once I reconnected both :

Or maybe somebody can tell me something different about those Sata ports?

 

[Krevnik]

Once the iMac was open and the motherboard was out, I discovered my Sata cable was partially disconnected from it : the big plug was still in (data, I guess), but the small plug was floating around loose (power, I think). Anyway, looking at the Sata cable, both are needed to make the drive work.

Or maybe somebody can tell me something different about those Sata ports?

Nope, that sounds exactly right. One is power, one is data, and if my memory isn't faulty, you are right about which is which.

 

[Nessdufrat]

So the apple genius was talking nonsense about this secondary Sata port, it was actually just the power port for the hard drive. Weird. It was written Sata on the schematics, though. But from the look of it, it was indeed a power plug. I guess I'll know in a few days.

No idea what more I could try to revive my lcd, by any chance?

 

[Nessdufrat]

Ok, some news about this computer: today, I reassembled it after putting a 12+16 pin SSD inside. I also put the Samsung pro SSD back inside, figuring that it might work this time now that the Sata cable is correctly plugged in.
Indeed.
I installed El Capitan on the motherboard SSD (calling it that to differentiate it from the other, more accessible Samsung pro SSD), I'll probably install Windows on the Samsung pro.
The fan is still acting crazy, which is weird, but I installed macs fan control and now it's fine.
I put the screen on top, haven't plugged it back in but I don't hold too much hope.

I do have a new question, though: I'm gonna need a hard drive for my data, and I was planning to use an external enclosure connected via usb3. The enclosure has its own power. Now, I plan on buying a 7200rpm hard drive, because that's what I always buy, but the question is: should I go for Sata III or is Sata II enough? I'm not sure a hard drive will really be using the full transmitting speed of the 6gbps, especially in a usb3 enclosure, but I'm not a pro. Before I buy something I don't really need, I'd rather go for the cheaper version than for a more expensive version that will be useless in my enclosure (like the Samsung pro 850 would have been in my usb 3 enclosure or thunderbolt 1 enclosure).

Thanks a lot for your answers!

 

[Krevnik]

It depends on the price difference, IMO. There are small advantages to going with SATA III, since the higher throughput also means lower latency on commands to the drive, which could help a bit with certain types of access. But if the price difference is big enough, I probably wouldn't sweat it and go with the SATA II.