RAID1 on Raspberry Pi fails. How can I check why?

I made a NAS server with samba on my Raspberry Pi4 with two external SSD devices, setup as a RAID1 volume. I used mdadm for creating the RAID volume:

sudo mdadm --create --verbose /dev/md127 --level=mirror --raid-devices=2 /dev/sda1 /dev/sdb1

By checking /proc/mdstat, it seemed to work at first, but later one SSD drive failed. Unrelated to this failure, I have changed my OS from Rasbian to the Manjaro OS for Rasberry Pi 4 and reconfigured the RAID1 Volume. I got the same error again. This at least shows that the error is reproducible on two different OS.

The SSD drives work fine, so the problem is not that one of them is broken. I can easily reformat them and use them as normal external storage for any other computer (Linux and Windows OS). I suspect, the problem is that the external drives (2x SanDisc SSD 500GB) do not have an external power source and that the Pi struggles with powering both of them?

Is there a way how I can check what the actual problem might be? Right now, I am just guessing what the problem might be and that's not very satisfying.

The output from mdadm is as follows:

sudo mdadm --detail /dev/md127

Version : 1.2 Creation Time : Tue Feb 11 10:36:00 2020 Raid Level : raid1 Array Size : 488251392 (465.63 GiB 499.97 GB) Used Dev Size : 488251392 (465.63 GiB 499.97 GB) Raid Devices : 2 Total Devices : 2 Persistence : Superblock is persistent

 Intent Bitmap : Internal

   Update Time : Sun Feb 16 09:05:57 2020
         State : clean, degraded 
Active Devices : 1

Working Devices : 1 Failed Devices : 1 Spare Devices : 0

Consistency Policy : bitmap

          Name : mkpi:PiRaidArray  (local to host mkpi)
          UUID : b19e319d:43c78d97:a0aba26a:24469c8f
        Events : 8849

Number   Major   Minor   RaidDevice State
   0       8        1        0      active sync   /dev/sda1
   -       0        0        1      removed

   1       8       17        -      faulty

And further, here is the ouput from cat /proc/mdstat

Personalities : [raid1] 
md127 : active raid1 sdb1[1](F) sda1[0]
      488251392 blocks super 1.2 [2/1] [U_]
      bitmap: 4/4 pages [16KB], 65536KB chunk

unused devices: <none>