Before starting a test session, I started a list of test to do in order toe xclude some potential causes, like boot on a different hard disk, don’t let the laptop fall into nap, etc.
the first one was done using the current El Capitan Public Beta 4 and it ran fine for a couple of hours until I fall into a nap and so does the OS X laptop.
When I awoke, I do the same to the laptop, let it time to really awake from the nap while I was drinking a glass of fresh water with mint syrup ;-
Back to the computer, I add some Rows filled some other Cells, and attempt to export the modified data: the place where the IOException arise: all was good in the western theater of operations.
I continue that for a while until I had to insert my infamous hard disk to take some more data from there
I let the computer fall into nap two or three more times, made some more export, data fillings and no more IOException.
My conclusion: I may a problem with my OS X installed SSD (either the SSD itself or the recently intalled OS) [i]OR[/i] El Capitan PB4 correct a default (bug).
Note to self (and you): an Apple SSD cannot be formated (low level format, multiple passes), but only cleared (even with Disk Utils). A boot on another hard disk, copy all, move to trash leaves the sSD in the same state Disk Utils do ! And Disk Utils do not report any trouble to me concerning this SSD.
What are your guesses ?