My backup script failed because the USB disk appeared under a different device node after a reboot. The script had one job and had apparently outsourced it to probe order.

I first added a loop that searched /dev/sd* and selected the first disk of roughly the right size. That is not identification; it is speed dating with block devices. One wrong match would turn a backup problem into a restoration problem.

The better route starts with the event. When the kernel discovers hardware, hotplug information reaches user space. sysfs exposes attributes and the device hierarchy. udev applies naming policy and creates nodes or symbolic links. A rule can therefore match something intrinsic to this disk, such as a serial attribute, rather than its temporary kernel name.

Before writing the rule, I inspected the sysfs path associated with the block device and walked upward to understand where the useful attribute lived. This distinction matters: a rule may receive attributes from the device itself and from parent devices, but blindly matching an attribute seen somewhere in /sys is an excellent way to create a rule that never fires.

The result was a stable symbolic link under /dev while the ordinary sd name remained available. My script uses the stable link and checks the filesystem before mounting it. udev supplies naming policy; it does not absolve the script from verifying what it is about to write.

I tested by unplugging the disk, attaching another one first, and reconnecting in different orders. Watching the hotplug environment and udev’s diagnostics was much faster than repeatedly editing a rule and staring at /dev with wounded optimism.

Cold-plug startup deserved the same test. A useful rule must behave consistently whether the disk is present during boot or attached later.

The division of responsibility is clean. The kernel reports what appeared and where it sits in the device model. User space decides what local name is useful. My machine’s nickname for a disk does not belong in a kernel driver.

Dynamic device management still has an intimidating number of moving parts, but the principle is reliable: match stable attributes, create a stable link, and keep transient enumeration out of scripts. Hardware order is an observation, not a promise.