This is the most I can get when plugging in a hard drive hot and only on some power connectors.
[71656.314271] ata5: exception Emask 0x50 SAct 0x0 SErr 0x90a02 action 0xe frozen
[71656.314277] ata5: irq_stat 0x00400000, PHY RDY changed
[71656.314285] ata5: SError: { RecovComm Persist HostInt PHYRdyChg 10B8B }
[71656.314294] ata5: hard resetting link
[71660.360686] ata5: softreset failed (device not ready)
[71660.360694] ata5: applying........
I separated the 2 drives in the RAID 1 array.
1 is the old one /dev/sda and is out of date, while the separated other one /dev/sdc was in another drive and mounted and used with more data (updated).
I wonder how mdadm will handle this:
usb-storage: device scan complete
md: md127 stopped.
md: bind
md: md127: raid array is not clean -- starting background reconstruction
raid1: raid set md127 active with 1 out of 2 m........