Expanded storage pool failed - degraded raid

[kuno]

I have a DS1522+. Unfortunately my expansion of the storage pool failed in a RAID 5. I have four 10TB drives and added a fifth and it failed. Now the RAID is degraded. I only used about 55% of my storage.

I can't get a correct replacement unfortunately. Is there a way to undo the expansion and go back to my 4 drives and fix the degraded status? Or will I have to re-do the entire raid array? I have my data all backed up.

For context I had 4 shucked drives. Got a fifth which failed. I tried to get a replacement but that drive is a slightly different 10TB model after I shucked it. 

Thank you!

[Grra]

I had a similar situation where a drive indicated imminent failure in my 12 disk array.

I replaced it with the one I had lying on the shelf for just such an occasion but after the rebuild I still had error messages.
(This is how my purchase of the DS1821+ was done sooner than expected.)

I was unable to resolve this and did not want to take more drastic action since some of the data was not backed up (due to storage limitation) to my 4-bay.
So I basically ordered a new NAS and copied over all the data to it.

Afterwards I did a factory reset on the 12-bay and afterwards it still works OK and is now the main backup NAS at home.

So perhaps someone that is more of a command line warrior can give you suggestions but if you have all your data backed up my recommendation would be just to do a factory reset and set up the NAS again with the new drive.
I will probably be less of a hassle.