"If someone today proposed storing data in a custom closed file format, on USB sticks, only readable on OS X, the pitfuls should be obvious."
Except that's what I see people doing. I read about a recent archival project. They said, well, we've heard CDs and DVDs dont' last long, we are going to use MO cartridges (magneto-optical.) So, these ARE rated to last a long time, but how many MO drives can you find now, and how many do you think will be left in 20 years given how uncommon they are now? They also say "Well, no Office files (which is smart), but decided also to not use plain ASCII, ODF, PDF, JPEG, etc., saying the specs may not be stable for 20+ years, in preference to making their own archival format." OK, so you are archiving data in your own proprietary format? In other words, to *make* things archival-grade, some now are making EXACTLY the same mistakes they made then.
I had one of those IBM floppy drives do *just* that -- plugged in an old PC with one of the full-height 5.25" drives, flames and smoke just *poured* out of the damn thing. I was chuffed to realize I was on the opposite side of the "U" shaped work area from the fire extinguisher, but it went out on it's own anyway. We have had several probably working ST-412s come in, but due to data disposal policies were required to send them out for disposal (IDE, SATA, or a few types of SCSI we'd wipe, the rest had to be physically destroyed.)