a lot of my external drives have this error Could not mount “BUFFALO 3TB”. (com.apple.DiskManagement.disenter error 0.)

Turn the Mac off, disconnect the drive, turn the machine on and reconnect. Then be patient till the drive reappears. It might still be faulty and if so, see my post in this thread of a month ago: "... I reconnected the faulty backup disk once more. After about 30 minutes, the Disk maintenance app came on, told me I could no longer write on the disk and advised to reformat it. This time reformatting worked and now I have two Time Machine backup disks in alternation—some degree of trust in Apple was lost for a long time though, I fear."

Hope this works for you, too!

Spotlight is problematic. A drive is dead while spotlight makes an inventory. This can take a long time (especially if you have a lot of data) while the external disc may not be available. You can turn off Spotlight (https://support.apple.com/guide/mac-help/turn-off-spotlight-suggestions-mchl62db64f5/mac) or through the linux window (https://www.technipages.com/macos-sierra-enable-disable-spotlight). Personally I never use spotlight. I have kept it enable and let it happily index for 12 houses. But this should be one your first go to if you have problems.

To clarify Time Machine backups. The fact Apple is asking if a drive is compatible with Catalina. Here is a thought don't make the drive I've used for 5+ years for Time Machine obsolete with a software update. Or make clear what no longer works with software updates.

Yes the hard drive has previously mounted to my mac. was last used on a mac but previously used on a windows and yes it supports all firmware updates. Please fix this issue as it's ridiculous that every time I want to access my backups I have to sit here re mounting trying to figure out what went wrong :(

Is there any solution to this? My entire backup is on this external drive. What the **** Apple! I can't spend hours going through techie posts. Is there a way to retrieve my data or is this going to require a visit to the Apple store. I've had more issues with my Apple products in the last few months. A time suck! It's why switched to Apple - to avoid hours trying solve techie issues.

Yes there is. I will comment more in a few hours hopefully when I get off the phone with Apple. What I have figured out is that Catalina runs a hard drive scan when it was removed improperly. So depending on the size of the hard drive you just need to leave it plugged in and wait 30-90 minutes and your computer will recognize it. The problem is that every the drive is removed and plugged back in it goes through the check again. That is what Im hoping to fix today.

So if you wait for it to show up in finder while logged in then you open migration assistant you have to wait again or if you boot to apple utilities to restore you have to wait again. Super lame but hope to have it resolved today.

Hi hapnet,

Thank you for using the Apple Support Communities! I understand you are running into an error when you are attempting to mount an external hard drive. I'd like to help.

-Have you checked in with the manufacturer of the external hard drive for compatibility with macOS Catalina, and also to complete any firmware updates that may be needed? If not, please do that.

-Has this external hard drive worked with your MacBook Pro previously?

-Was the drive last used on a windows computer or windows partition or parallels system?

Try running First Aid on the disk: Repair a storage device in Disk Utility on Mac - Apple Support.

If you need continued support, reach back out with an update on the issue, and responses to the above questions.

Thank you!

I have the exact same problem. Did you find a solution? Thanks!!

I have the same problem: two external hard drives, which I had been using for Time Machine backups. One had been used for a while, the other was brand new and had only been used for a day (first total backup hd been completed). Each separately suddenly disappeared after an error message that the Mac could no longer recover the disk but could read them and advised making a backup (haha!) and to reformat the disk. However, in reality it could not reven read them, and they do not appear in Finder anymore.

Compatible with OS Catalina? Yes.

Worked well previously.

Have not been connected to Windows Pcs.

Disk Utility notes an external hard disk, mentions even that it is formatted Mac OS Extended (journaled), but it is greyed out. Cannot perform any actions on it. When trying to Activate I get the error message as above: 'com.apple.DiskManagement.disenter error 0.'

What else can be done?

Thanks in advance!

Im having the exact same problem as hapnet and DonFW. 2TB WD Passport purchased from Apple store formatted from the manufacturer in Mac OS Extended (Journaled). It has time machine backup on it only.

Im using a Mac Pro Late 2013 (Trash Can)

Mac OS Catalina v10.15

drive worked fine an hour ago and was trying to eject it and the eject command wasn't working so I had to just unplug it. Now when I plug it in it does not show up in finder but shows up grayed out in Disk Utility.

The exact error I get when trying to mount through Disk Utility is: Could not mount “2TB Backup”. (com.apple.DiskManagement.disenter error 0.)

Have you tried a different port for the drive? I've begun running into the same issue and after some troubleshooting found that the Thunderbolt port on my laptop appears to be shot for any sort of data connection. Power goes fine and every now and then I can get a drive to connect, but it's always temporary and all of the same drives work fine in the other 3 Thunderbolt ports.

Yes, I've tried different port. The main reason I switched to Mac is I am not a computer tech person and got tired of scanning Google to fix computer problems. Yet, here I am again. I went to a neighbor who was able to retrieve my data on his Windows computer using Linux. What I have basically learned from this lesson is backups are not really reliable so I've at least moved my photos and documents to Google drive.

That worked! After a few weeks of cooling down (and buying another external backup disk) I reconnected the faulty backup disk once more. After about 30 minutes, the Disk maintenance app came on, told me I could no longer write on the disk and advised to reformat it. This time reformatting worked and now I have two Time Machine backup disks in alternation—some degree of trust in Apple was lost for a long time though, I fear.