How to check what piece of hardware is broken

How would I test which piece of hardware is broken: a sata to usb adapter or an internal harddrive? Sometimes, nothing is wrong at all. Other times, the harddrive keeps on disconnecting from my computer at random times, and im afraid it might destroy the platters from constantly stopping abruptly.

i dont want to have to buy another adapter/hdd to test out what is wrong. I want to buy only one object and be sure its replacing the right object

if this helps:
power cable: many, so i think that all of my power cables are fine

sata to usb cable: Newegg.com - SABRENT SATA-C35U Serial ATA (SATA) to USB 2.0 Cable Converter Adapter

harddrive: Newegg.com - Western Digital Caviar Green WD20EARS 2TB 64MB Cache SATA 3.0Gb/s 3.5" Internal Hard Drive -Bare Drive

I'm not certain that your problem is related to something that I read here on the forum recently or not. But WDC released a firmware patch for some drives, that would increase the amount of idle time before it would park the heads, reducing certain problems. I do not know that parking time would be involved with disconnections or not, and I failed to find exactly what I read. I do not think that this is exactly the patch mentioned, but it might help?

New Release - Universal Firmware Updater for Smart... - Western Digital Community

The harddrive is connected to my laptop's usb port with the sata to usb connector.

and @seekermeister: if my harddrive disconnects while updating, would that cause problems? also, my harddrive is encrypted, so would ANY and ALL programs that would require drive access during the update MUST be disabled . be a problem?

sorry for not replying sooner.

i unfortunately only have my laptop with me, so i cant test my hdd on another computer. also, it turns out that i have a seagate hdd. its kept in a cage that covers its top, and i had thought that it was a wd hdd. sorry

Suddenly everything is working again... i dont get it. i finally got seatools working, after not being able to use it every time i downloaded it before, and it shows the hdd working fine, so i guess its the cable...