T7500 Riser Card broken ?

Hi,

I have a T7500 server (or is it considered still a workstation?) which stopped one day with  "uncorrectable memory error in Riser DIMM 3". I tried reseating the memory without success (same error) and after a few changes to the memory ( it has 12x 4GB modules from Hynix 2Rx8 PC3L-10600R) it turns out it runs just fine when I remove all Riser Memory and only keep 6 Motherboard modules. My understanding is that this is not such a great memory configuration and so this morning after an overnight job which ran just fine, I removed DIMM4, 5 and 6 from the MB and moved it to Riser DIMM 1 2 and 3 which I believe is an acceptable configuration. A crash happened after about 10 min.with "Uncorrectable memory error has been previously detected in RISER DIMM 3" - This means that since the memory chip came from the MB that previously worked well overnight, it is not the memory that is broken but the RISER Card perhaps ? Is this right ? If so, can I plug a new one right in after I transfer the memory and the CPU from the old one ? Can this work ?

BTW, when this error occurs, the system offers me to hit F5 for tests: the memory tests run for a while until they freeze without any additional error message. At this point I can do nothing but reboot the hard way.

Thanks,

Lothar