My thoughts exactly. Well, not
exactly, my thoughts included a lot more expletives.
When the import failed on the new card, did you try attaching the array with your older card?
TLDR: migrating arrays from one LSI based RAID card to another is not 100% guaranteed; if the foreign import fails, don't force it; if you have to try and force an import, preserve any CacheCade arrays.
Longer version below.
WARNING: most of the conclusions below are based on nothing but some guess work on my part, YMMV.
If I had stopped trying to force things on the new controller, I'm guessing I could have swapped the old controller back into the system and continued as normal. While I didn't have everything backed up, all my critical data was backed up on a separate system so I thought I would play a bit ... this turned out to be a mistake, or at least my approach was flawed. I tried forcing an import of the CacheCade array using the approach described earlier in this thread (see the Broadcom/Avago/LSI support link I posted above), and that failed. I thought that the CacheCade array was periodically flushed to the backing arrays and on shutdown so using this array as a guinea pig seemed "safe". Unfortunately it wasn't. In fact I now suspect this was the worst thing I could have done.
With the associated CacheCade array now gone, even though the old controller recognized the arrays, it refused to start them without being forced (if you want details on how to force these arrays to start let me know, but if you are at that point you are already screwed). When I did get the arrays running the data corruption was substantial. The filesystems were in a bizarre state, likely caused by the mix of ages/validity of the drive blocks underneath them (CacheCade works at the device/block level, not the filesystem level). I was able to get some info from the arrays, but I'm considering all of it suspect at this point. I did grab an image of the arrays to see if I can fix some of the issues with an offline fsck, but right now that is more of an academic pursuit. At the moment I'm in the process of rebuilding the system and restoring my backups.
That's my tale, if nothing else hopefully it will serve as a warning to others - there definitely be dragons here!