SOLVED Strange behavior during hard disk burn-in

[Pitfrr]

Hello,

Note: This is not related to TrueNAS, even though I ran the test on FreeNAS.
I am facing a surprising (at least to me) behavior from a hard disk during (or rather after) a burn-in test.

Configuration:
System: FreeNAS 11.3-U4.1 running as VM under windows (I needed to have badblocks and I had this VM already configured)
HDD: WD160EDGZ (apparently not SMR), intended as backup (and spare), connected on USB and passed through to the VM

Testing:
I wanted to run burn-in tests on that drive.
So I ran a long SMART and a full pass of badblocks. Went fine, no errors.

Behavior:
What was strange is that as soon as I stopped the tests, the disk started to scratch (for lack of a better word) for about 80 minutes. Not a suspicious noise, it's like a normal HDD working but sustained for 80 minutes although the test was stopped. It was just unexpected.
I don't explain that, since the disk is not mounted on the system, so there is no access on it.
On the system's reporting page in FreeNAS, nothing is visible after 9:00 (when I stopped the badblock, it was starting again with a write pass)

Question:
Any clue on what might have happened?
This might be because of the setup? But I don't have the hardware to test with a direct SATA connection right now...
It gives me the feeling like some operations were buffered somehow and got executed at the end of the badblocks?

Thanks.

 

[Pitfrr]

I did a different test which might bring some explanation:

System:
Using an old FreeNAS 9.2 (just to be able to use badblocks) bare metal with a intel i5-650 and the disk connected directly to a SATA connector on the mainboard.

Testing:
Burn-in with badbloks and SMART tests

Behavior:
Unlike the previous test, when I stopped the badblocks, I didn't hear anything (or almost) from the disk (at least no heavy scratching like previously). So my explanation would be: I guess the USB controller added some caching or whatever... (so setting this thread to [solved])
Since I'm going to use this disk as backup and spare, I won't be able to see how it behaves in a pool right away.