Via 2 methods:
Method 1:
Prior to the drive finally getting kicked out and btrfs having a conniption there is all of this very many times in the syslog
Feb 17 12:47:03 Tower kernel: ata5: hard resetting link
Feb 17 12:47:05 Tower kernel: ata5: SATA link down (SStatus 1 SControl 310)
Searching back in the syslog for ata5 you wind up with
Feb 17 12:43:36 Tower kernel: ata5: SATA link up 3.0 Gbps (SStatus 123 SControl 300)
Feb 17 12:43:36 Tower kernel: ata5.00: ATA-10: WDC WD10SPZX-21Z10T0, WD-WX61A7852UTZ, 02.01A02, max UDMA/133
Cross reference that drive in the SMART section of the diagnostics and it doesn't exist at all (ie: no drive is connected)
But it was connected originally as the cache drive
Feb 17 12:43:39 Tower emhttpd: import 30 cache device: (sdb) WDC_WD10SPZX-21Z10T0_WD-WX61A7852UTZ
Which leads to
But, personally I think it was mostly method #2: