Readynas Ultra 4 RAID 5 - 2 of 4 disks fail at the same time

Before I was able to replace the disks (for which smart disk errors were reported previously), the next reboot gave me an unrecoverable disk problem (End of Life error). Checking the fs_check log, revealed a set of block reading errors resulting in short reads. It suggests to run fsck manually. (See end of log file below)

.....

Error reading block 242810730 (Attempt to read block from filesystem resulted in short read).  Ignore error? yes

Force rewrite? yes

fsck.ext4: Attempt to read block from filesystem resulted in short read while trying to re-open /dev/c/c

/dev/c/c: ********** WARNING: Filesystem still has errors **********




***** File system check performed at Sat Aug 25 19:18:09 CEST 2018 *****
fsck 1.42.12 (29-Aug-2014)
/dev/c/c: recovering journal
Error reading block 242799888 (Attempt to read block from filesystem resulted in short read).  

/dev/c/c: UNEXPECTED INCONSISTENCY; RUN fsck MANUALLY.
	(i.e., without -a or -p options)

However when I run fsck with -v -n to get more information, it does not get past de short read error. (See below)

/root$ fsck.ext4 -v -n /dev/c/c
e2fsck 1.42.12 (29-Aug-2014)
fsck.ext4: Attempt to read block from filesystem resulted in short read while trying to open /dev/c/c
Could this be a zero-length partition?

How can I run fsck manually in the correct way, to attempt to identify an possibly resolve the disk errors.

Any suggestions are apreciatied.

Regards,

Mark