Just for the record—a bad SATA power cable is capable of provoking
intermittent errors such as the following:
ata4: exception Emask 0x10 SAct 0x0 SErr 0x4050002 action 0xe frozen
ata4: irq_stat 0x00400040, connection status changed
ata4: SError: { RecovComm PHYRdyChg CommWake DevExch }
ata4: hard resetting link
ata4: SATA link down (SStatus 0 SControl 300)
ata4: EH complete
Two chassis fans and a disk were sharing a single 4-pin molex connector
from the power supply. The problem was that the pin sockets on the fan
connectors were loose and misaligned, and one of them became dislodged
when the disk connector was plugged in. Pushing the pins into place and
reattaching the connectors solved the problem.