Physically Identify the failed hard drive

So, let's say your server had 6 healthy hard drives. A drive fails (will not mount/detect, drops out of raid with errors) or is failing (SMART getting worse, etc). You need to swap out the bad drive. When you open the case you see.. six identical hard drives.

How can you tell which one is no longer healthy/mounting/functioning?

System would be linux, most likely ubuntu server, using at most simple software RAID. The hard drives would be SATA and connected directly to the motherboard. (no raid controller)

I don't want to randomly disconnect drives until I pick the correct one. The drives all appear identical to me; I imagine there is some common way to identify which drive is which that I am unaware of. Does anyone have any pointers/tips/best practices? Thanks!

EDIT: I had wanted this to be 'generalized' in a hand-wavy sort of way, but it just came off as 'incomplete' and 'horrible'. My bad!

I had this exact problem on a (tower) server just like you explain, and it was easy:

smartctl will output the serial number of the drive

Vendors sometimes ship their own specific tools, like hdparm, that will do the same.

So output the serial of the bad drive, and then use a dentist's mirror and a flashlight to find the drive.

On a rackmount you'll usually have indicator lights like other people have said, but I bet the same would apply.

Putting stickers on drives (depending on the design of the tray) may not be feasible. By the time the drive dies, the stickers could be dried up and fallen off.

ledctl (from package ledmon) is really the way to go with this.

ledctl locate=/dev/disk/by-id/[drive-id]

or

ledctl locate=/dev/sda

will illuminate the drive fail light on your chassis for the specified drive. I provided two examples to illustrate that it doesn't matter HOW you identify the drive. You can use serial, name, etc... Whatever information is available to you can be used. The drives are referenced multiple ways under the /dev/ and /dev/disk/ path.

To turn the light back off, just execute it again, changing locate to locate_off like so:

ledctl locate_off=/dev/sda