I guess that makes you one very lucky user.. or unlucky as you need to discover whether Storage Spaces can be migrated to 1709 successfully- and what to do to deal with this problem in general.
Suggest you take it up on technet (MS site) and report via the Feedback Hub app.
Meanwhile you need to consider your possibly dodgy disk - try a surface scan- and of course appropriate backups.
As you have Pro, you can use
to delay 1709 for a year
Glad to hear your data are available again, but because we didn't identify the cause of problem, you have to make backup before next attempt to v1709 upgrade. Anyway, you can defer this upgrade for one year.
Regarding cause - personally I suspect either wrong disk controller driver or some inconsistency of existing RAID metadata, which manifests itself in version 1709 only (because I made three successfull OS upgrades with Storage Spaces RAID retained - two 3disks/Parity in test environment and one 4disks/Mirror in my workstation). With all data backed up (twice), you may remove existing RAID and create it again from scratch. All three harddisks IMO are physically OK, but you should monitor possible changes in their SMART state.
Haven't left you guys yet. Just an update here, I did try the v1709 upgrade again once my data was well backed up and again I encountered the same problem with my storage pool. Next I may try tearing down the pool, upgrading, then recreating it under the new Win10 build.
It seems that there is something wrong with your Parity RAID... Back it up twice on different media, remove existing Storage Space completely, re-create it again, restore data. Then it should survive upgrade to v1709 without problems. (Don't forget to write down which SATA controller driver is used before and after upgrade; it may interfere with Parity RAID too.)
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