How to connect my RB5009 to my CRS328-24P-4S+ (within the same rack)?
I'm up and running with standard Cat6, but I'm trying to learn about SFP modules and which to use. Looking at the list of them on MikroTik site is intimidating and unhelpful. I don't know where to start or what to look for. Copper vs Fiber? DAC? What are all the dozens of different models and what am I looking for? Is there a guide somewhere?

Sounds like
a. a simple Fiber patch cable once you decide on format
b. for short distances within a cabinet i believe you can use a MM connection for both ends with 10gig connectors.

LC to LC connectors and match up with a pair of transceivers of the same ilk.
I would go with something like

patch cable 2m (6.6 feet)
https://www.amazon.ca/Cable-Matters-Mul ... r=8-6&th=1

Transceiver module
https://www.amazon.ca/10Gtek-SFP-10G-SR ... r=8-7&th=1

which by the way I believe makes this model branded for MT
https://mikrotik.com/product/Splus85DLC03D

I can confirm that those exact 10gtek transceivers from Amazon work with my RB5009, just set up a pair with a long fiber cable to my NAS in another room

Suggestion by @anav to use optical interconnect is great. Avoid copper connections (Cat6 or DACs) because it seems that more energy is needed to push data at high rates over copper than over fibre - RJ45 SFP+ modules tend to run very hot and can cause various problems. Copper DACs are somehow better, but come only in short sizes and can cause incompatibility problems ... fibre connectiobs is slightly more expensive, but is highly flexible (each connection point can use different vendor of SFP+ module for higher compatibility with device, cable length can be much longer, ...), you only have to be careful about optical interface compatibiliry of the connected modules (single-mode vs. multi-mode, wavelength when using SM) and to use appropriate connecting optical cable (SM vs. MM, connector type aporopriate for each module).

Thanks for the info and recommendations! I’ve put in my order and looking forward to testing it out. Thanks.

Well, got the kit, but I can't seem to make it work. There is a connection, but it seems to be one-way. RB5009 is TX but not RX; while the CRS328 is RX but not TX. I've tried with auto-negotiation on/off, different speeds, but it's only one-way. RB5009 shows status as Enabled / Running / Slave / Link OK / Auto-Negotiation Incomplete / Rate 10G / Full Duplex. CRS328 shows Enabled / Running / Link OK / Auto-Negotiation Done / Rate 10G / Full Duplex. I've tried switching the cable around, switching the transceivers around, rebooting both devices. Same results every time.

Are there settings that I've missed? Do I have faulty equipment? Switching the transceivers around would seem to eliminate them, right? Maybe bad cable? More likely, I've done something wrong or not done something that I should have....

I can't if you have a pair of fibers. If you have a SFP with an Rx and Tx then you have swap the fibers at one end.

RX -- TX
TX -- RX

I can't if you have a pair of fibers. If you have a SFP with an Rx and Tx then you have swap the fibers at one end.

RX -- TX
TX -- RX

The transceivers I have are the 10gtek suggested by anav above. The cable is a monoprice cable (https://www.monoprice.com/product?p_id=6385). The cable does crossover from one end to the other as you can see from the photo at the link. If they are swappable, I can't tell. But they do swap position from one end to the other. And this seems correct with the results I see. The RB5009 TX is going to the CRS328 RX. But the CRS328 is not TX anything and the RB5009 is not RX. Why is the CRS328 not TX?

Looked in this forum and the 328 has problems with SFP under ROS7. If you are on that version then try if 1G speed works.

The cable pictured swaps, so that is good.

Looked in this forum and the 328 has problems with SFP under ROS7. If you are on that version then try if 1G speed works.

The cable pictured swaps, so that is good.

Hadn't seen that yet on the forums, but I did just try connecting RB5009 to my CRS112 and it does work; no auto-negotiation, 1GB. But no setting seems to work connecting RB5009 to the CRS328. That is extremely frustrating. Wonder if I should try rolling back ROS version...

I hear you. Yes, it may be worthwhile seeing if that helps.
I imagine you played with all the settings, set speed to auto, or negotiate etc etc........

Saw this in another post.....
"Lots of us are having problems with SFP/SFP+s in 7.x. Mine have link failures every 5 minutes (10G SMF fs.com parts) that can only be rectified by running them forced at 1G. I’m hoping the next release(s) resolve this; if you look in the announcement thread for 7.1.1 and 7.2rc1 you’ll see there are many support tickets filed related to this."