How do I ensure a USB external HDD gets assigned the same SCSI id??

keenboy · 02-03-2009, 08:51 AM

I have 3 WD External USB hard dives. 2 are the same and are rotated for backup purposes and my system assigns them both to /dev/sdb (one is only plugged in at a time), this is fine.

The 3rd drive is used for a different purpose and may or may not be plugged in at the same time as one of the others.

How can I make the 3rd drive be allocated to /dev/sdc all of the time whether one of the other drives is plugged in or not?

Thanks

Nylex · 02-03-2009, 09:56 AM

You should look into writing udev rules for your devices.

David the H. · 02-03-2009, 10:04 AM

You can use udev to set up hardware-specific devices. Simply add a rule to have it symlink the device to a particular /dev entry. It's probably better however to create and use a dedicated device with a memorable name like "backup_disk_a" than to try to mess with the traditional sd* devices.

And actually, udev has already set up some links for you. You can find permanent symlinks to each block device in the /dev/disk/by-whatever directories. You can use those to access the device you want, no matter what the /dev/sd* number is.

Didier Spaier · 02-03-2009, 10:08 AM

I would try to write an udev rule for (re)naming it, based on its characteristics.

bwayson · 02-03-2009, 02:50 PM

If your goal is to always know where to access a drive, the /dev/disk/by-whatever method is good. Another simple way to do that is to give the disk a label. mlabel will label a disk formatted in some fat format, e2label will do so for ext2 and ext3 formatted disks. Most distros I've seen will dynamically create a folder under /media of the same name as the label and mount the disk there whenever it is plugged into the PC. Your only concern is keeping the label names unique.