I stumbled into a much longer thread on this subject here somewhere, but I can't find it now...so here goes.
I just successfully did this on my two older Ultras, one is a 4640 and the other is a 4670. I read here about the woes of some who have had difficulty getting their Ultras to recognize the cards they used. The entries on that longer thread ultimately sussed out that no matter how large the capacity of the card you choose, the Ultra will/can only make use of 2G of the total volume, even though it will be aware of how large the card actually is. With that 2G cap in mind, I ordered a pair of these (they're sold in pairs):
https://www.amazon.com/dp/B08L8RC2Y2?psc=1&ref=ppx_yo2ov_dt_b_product_details
They were less than $10 for the pair. I figured, why waist a lot of money on more expensive, larger cards when the Ultra will be perfectly (and possibly, only) happy with just 2G. I did go way overboard (😉) and spring for the extra $0.09 for 4G cards vs the 2G cards. I couldn't help myself. Anyway, as soon as they came, I followed the thread suggestion that the card should be inserted (shiny contacts facing up) with the Ultra already turned on. I did do a fresh reboot on each unit before inserting the cards, though. In both the 4640 and 4670, the units recognized the cards as soon as they were inserted and immediately asked if I wanted to format them. Yes, please! It was a slow format, but with a count-up percentage displayed, just like Ultras do with anything loading, etc.
Afterward, just for thoroughness, I rebooted the units again, and then proceeded to activate each of my 18 loaded apps. This was the last time I saw any "Loading" messages on any of the apps (well...Netflix almost always says "loading" very briefly). Now, all the apps just load. Interestingly, while the 4670 reports using 0.2G of 3.9G, the 4640 reports using 0.3G of 3.9G. I don't know why there's this slight discrepancy...unless the 4670 has a bit more internal storage than the 4640. 🤔
I guess my main point is: don't over-think this process. I understand the temptation is to lean toward larger, bleeding edge cards...but these older Ultras just don't need, and can't make use of all of, them anyway. Force yourself to go simple, cheap, small, and you'll be just fine. And, contrary to conservative common practice, plug the card in while the unit is already turned on. Apparently, it's often the only way the Ultra will initially recognize the card.
I hope this will help someone. Happy streaming!
