I was able to get a great deal on a 5210 with the 1gb cache module, and battery for 150 from an Amazon seller, no less. I messed with it a bit, then put it aside, not wanting to screw anything else up until I built another system around the 12gb SAS.
Issues I had:
1) would NOT recognize arrays from a 2208 or 2108 (either one)
2) had a heck of a time figuring out how to get them recognized once switched back to the original controller (Intel SB008 aka 9286cv-8e)
3) so I moved on to the keys issue... Hardware key from 2208 (oemxs Intel ssd2 type) does not work- no surprise there
3a) but what I really wanted was Raid6-- which you have to buy from IBM.
3b) so... Ordered R6 key from Provantage for $60 or so-- but it does NOT work like an LSI/Avago upgrade, you have to basically have it in one of several IBM models to install the FOD-- of course, I don't have one of those models.
3c) so, the instructions say to email IBM... I finally gave up and went back to the 9286.
That said, if anyone does try to cross flash, or has even limited success, I would really appreciate knowing... I was unable to find much of anything on the web, re: this.
In an odd twist of fate, the R6 code they sent me is supposedly good for 8 activations, tho it seems to eat one activation every time you enter it to go to the system selection page. If anyone manages to figure out the FOD issue, I'd be more than happy to share the code if I can get mine updated/upgraded. (That's likely enough to go around....)
Of course, cross-flashing will likely wipe out the R6 feature anyway, so am not sure it matters.