different ways to determine which disk is failing

source : debian-user-digest # 1207,1208,1209.

[ A ] : cat /proc/mdstat can help but you need to get the serial numbers. Do this :

# hdparm -i /dev/sda

[ B ] : Are any USB communication errors being logged along with the md and ext3 errors ?
Are you sure it's a disk drive problem and not an issue with the kernel drivers, system BIOS, USB controller, cabling, or a combination thereof ?
How long ( days, weeks, months, years ) did this exact setup function properly
before you started seeing these problems ?
Did you recently perform any major software upgrades ( kernel/drivers ) shortly before this problem surfaced ?

[ C ] : you could also try smartctl -a /dev/sda to get the disks serial numbers.

[ D ] : You might try lsusb - to list devices on your usb bus. That might help you identify specific devices.
Also try nosing around in the sub-directories under /dev/disk.
And, perhaps an obvious question, but does the drive maker provide any device-specific drivers or utilities that might help ?

[ E ] : Both external drives are native SATA correct ? I think your best course of action at this point would be to purchase a $15-20 two port PCI SATA card based on a SiI 3512 chipset, any internal SATA data/power cables you'd need, and move the drives inside the PC. This will allow smartmontools, hdparm, and other utils to identify the drives, and you'll likely get a nice speed boost as well, especially if that PC has a 66MHz 32bit PCI slot, which will allow full bandwidth to both drives simultaneously.
 
 

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