I'm pretty sure that the reconstruction will have covered all the data (it wasn't completely full, and assuming some kind of defragging or logic when writing to the disks in the first place, my data should have all been reconstructed already
Unfortunately that's not how it works. The raid level is below the filesystem, and so it doesn't know which sectors are files, and which sectors are empty space. When resyncing it will just start at sector 1, and go on until all sectors are covered.
And no, the filessystem doesn't fill up the space linear. It will put the files on 'random' places, to prevent fragmentation. When the resyncing was done for 90%, you can assume 10% of your files are not yet covered.
Now I'm not 100% sure the disks have actually failed, as I dropped the first 'failed' one into a SATA caddy after it was replaced, and it showed me the partitioning table etc.
It failed because it got an I/O error while reading the disk. AFAIK that is the only
reason why Linux software raid drops a disk. It is possible that there is only a single bad sector. In normal use you won't notice that.
1) Is there a simple way to get a command line on the 5Big v1, and if so, has anyone written a definitive guide on how to do this without faffing a large amount?
You can try the tricks for the 2Big Network
2) Assuming the disks are actually 'usable' still (ie not totally catastrophically dead), is there any way to 'force' the failed disk to appear as good for just long enough (read: 2 hours or so) to get the reconstruction of the 'new' disk completed?
Yes and no. You can force a failed disk back in the array, but AFAIK you can't continue the resync at 90% then. You'll have to start over again.
The trick is to rebuild the array with exact the same settings
, while telling the system that the array is clean. More info here
3) If the answer to 2) above is 'no', is there a way to force the 'new' disk (ie the one that was rebuilding itself into the array) to show as 'rebuilt' in order to assess the level of data loss (if any) that I've actually sustained?
Yes, the same way. But as 10% of the surface is not rebuild yet, odd are that it won't mount. It's like removing 10% of all shelves in a store, and hope you'll still be able to find your stuff without stumbling.