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@Stewart Fleming - Well the good news you have two drive ports! A 3.5” SATA drive port and a PCIe Blade SSD drive port on the back side of the logic board. The system was sold in three different configurations SATA HDD or PCIe SSD only or the two together which is what a Fusion Drive is. So in your case it sounds like you got a PCIe blade SSD based system. You can get the needed SATA cable to add in either a 2.5” or 3.5” HDD or 2.5” SSD. Here’s the needed parts: 27” iMac Retina 5K Hard Drive Combo Cable (L15/17/19), Apple P/N 923-00664Hard Drive Mount Left, Apple P/N 923-00086Hard Drive Mount Right, Apple P/N 923-00087iMac 27" Screw Set - Hard Drive Mount Left (x2) & L/R brackets (x4) six total, Apple P/N 923-0331iMac 27" Screw Set - Hard Drive Mount Right (x2), Apple P/N’s 923-0331 & 923-0396NewerTech AdaptaDrive 2.5 to 3.5inch drive converter Here’s the guide to put in the SATA drive iMac Intel 27" Retina 5K Display Hard Drive Replacement Its the same for your 2019 system but you still need to reach around to plug in the cable as the connection is on the back side. Your other option which I don’t think you need is swapping out your current PCIe/NVMe blade SSD with a larger one. Here’s that guide iMac Intel 27" Retina 5K Display Blade SSD Replacement Unless you are working on very large data sets (CAD, Animation, layered drawings, Photos, Videos) you shouldn’t need anything larger than 512GB that you have. Whats important is how you setup your drives. Here the blade SSD is setup as your boot drive (macOS) and holds your Apps. Leaving as much of the drive empty! So your OS can use it for virtual RAM, caching/paging and depending on your apps scratch space. Your second drive is your data drive holding all of your media (music/vids) and your work.

For your question: What I am curious to know is if there is room inside to install a 2.5” SSD drive ? Your iMac had to come with either a fusion drive or a HDD from the factory to have both the SSD blade and the 2.5” SSD drive. If it only came with an SSD, it’s not reasonably possible unfortunately to add the 2.5” SSD. In the end I always say everything is possible, but I think it’s more than a bracket that you would need. So for 99% of folks, including the ones on iFixit, it’s not possible. I always buy iMac’s with a fusion drive and then I can add whatever I want later.