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Apr 23 15:12:13.064341 LEMC kernel: EXT4-fs (sdb1): error count since last fsck: 13
Apr 23 15:12:13.064824 LEMC kernel: EXT4-fs (sdb1): initial error at time 1598463659: ext4_validate_block_bitmap:384
Apr 23 15:12:13.065026 LEMC kernel: EXT4-fs (sdb1): last error at time 1599941347: ext4_free_blocks:4964
Both drives were fsck'd on boot due to dirty filesystems (not unmounted cleanly before shutdown) but it's impossible to see/say what causes the shutdowns. The ^ above errors suggest the drive has errors after being fsck'd so it might be dying. The largest cause of drive mounting issues and instability on RPi boards is an inadequate power supply. And power characteristics of the board under LE11 may also be different than LE9 due to major software changes and general evolution. If you're adding drives then the board is going to draw more current even if the drives are powered from a separate hub or via their own PSUs.