Hi, Actually I have the original 5400 RPM 2.5" 1TB HD in my 21.5" late 2013 IMac model 14,1 EMC 2638 I am looking for advise to decide which type or which combination of SSD units is worth for me since I have an internal PCI express port so the three options are: 1) 2.5" internal SSD 2) PCI express SSD 3) External SSD I already know how to teardown my computer when I I upgraded the original 8GB memory to 16GB so open it again is not a problem in fact it's exiting I bought my IMac just to make music, I installed Logic Pro X and more than 300GB of virtual instruments, as you know those type of libraries are very large, I have libraries from 16GB to 57GB and also these virtual instruments are good to save them in a much faster drive so I thought to place a 256GB or 512GB PCIexpress SSD to save those libraries and keep my HD for other apps. By the way I don't know where to get it but from Apple Service Center. On the other hand I could place an internal 2.5" SSD instead the actual HD and use as I use my HD, I mean for all but I would need 1TB and it is very expensive for me. And the last option would be to buy an external SSD to place 300GB of virtual instruments and digital music stuff and use my HD to place te OS, the apps and other files. Any advice is very welcome. Thanks!