MacBook Pro: Second HDD Caddy - mounted SSD disappears after a while

MacBook Pro (13-inch, Early 2011), latest macOS.

I installed a 2nd HDD Caddy into optical tray and inserted an SSD (256gb Samsung). My primary is 500gb SSD (Samsung EVO).

After a while a mounted SSD stops to work. (Though I could download about 50Gb, lets say before disappearing.) Restart helps: my old SSD becomes visible.

I tried another 2nd HDD Caddy. I tried another disk, HDD. Same problem: after a while it disappears.

You can not see drive in Disk Utilities. The cable from caddy to MacBook is the same as was for DVD drive.

Please help!