PERC H700 and HP 468406-B21/487738-001 SAS Expander
This is just a micro-article because I didn't find this info anywhere on the web, so hopefully google picks this up for one or two other people scratching their head.
I just plugged in my HP 24 port SAS expander into one of my Dell PERC H700 controllers, and instead of the BIOS screen, I (roughly) got the message "
Unrecoverable Error!!! Please check the SDRAM connection" and it didn't work. Very scary - unplugging the expander made it work again.
When plugging it in after booting Debian, I got this kernel message:
External Enclosure Pd 20 violation detected in Direct PD Mapping mode
Since googling this message didn't help much, I had to use my own mental resources to solve this problem (usually thats futile), but after staring at the message, and remembering a passage from the manual, I got an idea and disabling direct PD mapping in the controller, because you shouldn't have it enabled when using enclosures:
perccli /c0 set directpdmapping=off
After rebooting the controller booted normally and started to detect disks behind the expander.