MDADM Raid6 Array only able to read parts of files

I run a ubuntu nas with raid6, mdstat does not indicate that anything is wrong, but when i try to read files on the raid volume, i am only able to read about 1/4 of the file, and the file transfer fails/freezes. I have tried multiple ways of reading files on the array (copy, ftp, http lamp server, but it always fail. The same happens to every file i try to read. The raid array have been running for months with no problems, and i cant seam to find what is wrong. I hae also done a memory test which found no errors. Could this be dead harddrives that have not been detected by mdadm?

Here is the output of mdstat:

Personalities : [linear] [multipath] [raid0] [raid1] [raid6] [raid5] [raid4] [raid10] 
md127 : active raid6 dm-2[1] dm-3[0] sdd1[4] sdb1[5]
      5860267648 blocks super 1.2 level 6, 64k chunk, algorithm 2 [4/4] [UUUU]

unused devices: <none>

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