BU40N can’t mount one disc unless flipped/vertical

[Dino-Killer 912]

I want to share a weird BU40N case in case it helps someone else. I’m posting the full context + what finally fixed it.

Drive / setup
Drive: HL-DT-ST BD-RE BU40N
Firmware: 1.03 (LibreDrive patch enabled)
Platform shown by MakeMKV: MT1959
OS: Windows, USB slim external use
LibreDrive features all enabled

Symptom
One specific BD-25 (Hong Kong release of Rigor Mortis) would not mount on this BU40N at all.
As soon as I inserted the disc, the drive made a regular clicking / seeking sound (“clack-clack-clack” on a steady rhythm).
Windows drive letter never changed to show disc capacity.
Other discs all work fine, including lots of BD-25/BD-50 and UHD BD-66/BD-100. This was the first disc out of hundreds that behaved like this.
I first assumed the disc was defective and even bought a second copy that is from a different pressing batch. Same behavior on BU40N.
Then I started suspecting the drive or LibreDrive patch / firmware.

What I tried
Cleaning the disc carefully.
Different USB ports / direct connection (no hub).
I was about to flash back to stock firmware, but I hit another issue: SDFtool “READ firmware” stopped after a few seconds, so backup wasn’t happening reliably.
Before dealing with the backup issue, I decided to try something else.

The surprising fix
By pure accident I tried changing the physical orientation of the drive:
Vertical orientation (either left-side up or right-side up): disc mounts normally.
Horizontal but flipped 180°: disc mounts normally.
Horizontal in the normal orientation: disc fails 100%, with the same rhythmic clicking and no mount.

This is extremely consistent/repeatable:
Normal horizontal = always fails
Flipped horizontal = always OK
Vertical (both sides) = OK

Conclusion
This appears to be a direction-dependent physical tolerance issue (disc slight imbalance/warp/centering edge case + slim MT1959 mechanics with narrow initialization tolerance), not a LibreDrive or firmware problem.
The disc isn't dead” it’s just right on the edge so gravity/force direction decides whether lead-in/TOC focus lock succeeds.

Why I’m posting
I wasted a lot of time (and money — Both copies were bought on a Chinese second-hand marketplace so I even argued a return and lost ~US$10) thinking it was a defective disc or firmware issue.

So if anyone sees:
slim BU40N / MT1959
one specific disc
steady rhythmic clicking on insert
no capacity/mount

…try flipping the drive 180° or standing it vertically before going down the rabbit hole.


Hope this helps someone!
If there’s any known BU40N tolerance note about direction-dependent initialization, I’d love to learn more.

[Billycar11]

Never move the drive while it's on had lot of people do that when I was first selling bu40n and it does not turn out well it will scratch your discs and your laser it will be dead

[Dino-Killer 912]

Never move the drive while it's on had lot of people do that when I was first selling bu40n and it does not turn out well it will scratch your discs and your laser it will be dead

I think moving the drive while it’s on could cause a read failure. Anyway, in my case, I made sure the drive was stable—either standing upright or lying flipped—before inserting the tray.