I am going to assume based on your references to the "Upper SATA Cable" That you intend to install a 2.5" SATA SSD since that's the style which would connect to that cable. I'm not sure it matters, ultimately since PCIe to SATA is not just a simple conversion, Desktop PCs make use of an entire card for this purpose. But other drive form factors which use the SATA standard exist, so I wanted to make sure I understand correctly. [br]
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I have never seen that sort of converter, or adapter. If you want to install a second drive, then your only option would be to install a compatible drive. So yes, realistically OWC, or an m.2 drive via an adapter are your only options. Though if you wanted to try for the adapter method, which could save some money I would do research as to your best options. There are a bevy of substandard adapters and drives.