Monterey issue with external HFS+ disk. It is NOT a HW issue.

Monterey has been a mess. You never know what might break with each update. BTW, Monterey is now as version 12.6.

Which OS did you use with First Aid that reported the b-tree error? Did you try running First Aid from the other OS as well? It is not surprising First Aid is unable to fix things....it has always been terrible at repairing its own file system. Also, always click "Show Details" and manually review the report since the First Aid summary many times will show "Ok" even after discovering some crippling unfixed errors (I guess this is what "Think Different" means).

FYI, you can try running the paid third party app Disk Warrior which is known to fix many file system issues with HFS+ volumes (does not repair APFS volumes since Apple has not released the necessary APFS documentation).

You can always provide Apple with product feedback here (Apple won't reply to you):

Product Feedback - Apple

The only time I have ever seen Disk Warrior not repair the HFS+ file system is when there is a hardware issue (many times DW will actually say this). Definitely run DW from the OS which is able to use the drive properly.

Run DriveDx to attempt to check the health of the drive. You will need to install a special USB driver. However, even with a special USB driver some drive enclosures will not allow the necessary communication to access the health information on the drive. For hard drives "Warning" or "Failing" notices indicate a worn out drive or a failing drive respectively. These types of apps can be misleading when used to check SSDs since "warning" & "failing" notices don't always mean the SSD is bad.

I now back up EVERYTHING to cloud backup ( not Apple) aftef rescuing a 2TB of photos frysn external drive that Monterey corrupted. We continue to use Monterey but unplug the external drive when not backing up files to it. I am highly skeptical of Monterey and will never buy another Apple computer. I’ve never had a problem like this with a Windows machine. J know this is heresy to Apple fans, but until they own up to the problem lost faith

David Kindler wrote:

But others have had this serious issue. So in the spirit of helping just explaining what happened to me

That’s what makes it weird. I have a myriad variety of external drives connected and don’t have problems with any of them. Since I don’t install any system modifications, but many people with problems posted here seem to have many system modifications installed. I would look in that direction for a solution.

Absolutely infuriating. I almost lost 2TB of data from my external HDD when Monterey did sn upgrade. Same story- drive couldn’t mount. A friend an IT pro saved me with Disk Drill. So far over 100 hours of recovery time. I no longer trust Apple. That’s my last Apple computer. Never had such an issue with a Windows machine. Turned off automatic upgrade on Monterey. Backing up everything (guilty of not watching my cloud backup more closely when it stopped grabbing files off the drive that subsequently had the mount issue)

Apple - are you listening? Do you care?

Apple no longer listens. I created a ticket against Apple two weeks ago. No reply.

They have never replied to much of anything in their 30+ year existence.

Which USB are you using? I have a myriad of USB3, HDD (both HFS+ and APFS), and SSD (APFS) attached to USB3 w/hub and TB2, 4-disk arrays to an M1 Mini. I have had no problems with any of them.

The HDD's are Toshiba and Seagate. The SSD's are all Kensington.

I don't think any are using USB through the USB-C ports.

I use USB3. I have a USB3 HD dock.

Weird.

I’ve never had a problem like this with a Windows machine.

Strange, I’ve never had that problem on any Mac. I have had many problems with Windows. Must be one if those weird karma things.

But others have had this serious issue. So in the spirit of helping just explaining what happened to me