Change from 1Tx4 to 2Tx4 dataprotection

I upgraded my mini disk space from 1Tx4 to 2Tx4 by swapping one disk at a time and letting it run over night. Now, every time I try to use the mini, it is very slow to copy any data onto it, and after a short while I get a disk error. After that it usually goes into dat protection mode and is totally unavailable for a while. I still have the original drive, but have no idea which slots they came from. Need to access my data. Run Mac OS.
Firmware 3.5.0 [7.171.80644]
Any way to fix this or get back to my the data on my old drives?

hi rolfoetter,
can i check how much data you have on your drobo (and approximately how much free space in values and % you have, if possible) - and how many blue lights are lit up on your drobo and dashboard.

if you can also have a look in dashboard, do any of the drives there show a label message, such as Warning or Healed?
(it may unfortunately be a drive failing, possibly if it was old, but even a new drive could encounter issues)

one thing you could do, could be to raise a ticket with the support team (which i belive can still be created as a 1 time paid ticket, even in case out of support), and then support team could use some diagnostics logs to try and determine which (2TB) drive is currently faulty, with a remedy etc, but please do not try to generate any diagnostics logs yet, in case they need to be generated at a specific time.

another method could also be to try putting your drobo into a readonly mode, for example as mentioned below, which might help to stabalise the drobo to at least try to copy your data to somewhere else, though it would not fix the actual root cause of the problem, but at least hopefully can stabalise it for accessing the data if you need it quickly: