External hard drive won't mount in high sierra

Had similar issue. Open disk utilities. If none of these drives show up look for the menu (new in High Sierra) that states "show all". If drive shows up try repair and if that does not work you may need to reformat it if nothing on that needs saving. I had two drives that I needed to fix that way. They were both fine with Sierra but High Sierra must be doing something different.

It seems a lot of people having the problem have their time machine backup on the drive, as me. I tried to remove "dataless" snapshots as explained in this article and i could mount my drive again afterwards : https://frenchw.net/en/tags/timemachine-en/

Many thanks. I solved the problem first by going back to my old MacBook Pro with an old version of OSX. In DU it showed all parts of the drive (by default) and not just the volumes. While it was being fixed I read your post so hence I have switched to "show all devices" on our other Macs.

All of my external drives worked fine until I upgraded to High Sierra. After the upgrade, the only drive that would show up was my older WD external (USB 2.0). I have a 3.0 USB caddy with a Hitachi and Seagate drive that appeared to be invisible to my iMac. Nothing I tried would make them show up and mount. After a few weeks of reading these forums with people having the same issue and desperately looking for a solution, I decided to plug my 3.0 drive caddy in to my powered USB hub (also older and not 3.0) and the drives mounted with no problem. It would appear to me that High Sierra has a problem with the USB 3.0 protocol.

I have had this problem everyday, and I have found a few temporary "fixes".

1. Reseting your USB - Do this by restarting your computer and immediately holding down 'Command, alt/option, p, r' once the scene goes black. Your computer will doing its usual ding, light up, then power down and ding again. Then release the input. I have found this works 2/3 of the time. Sometimes I have to do this twice, or it doesn't work at all

2. Plug in a USB 2.0 Hub and then plug in your drive. For some reason this has worked for me. My issue with this is I am editing 4K files and USB 2.0 is slower, but it is better than nothing. I have found this also does not work when your plug your drive into your extra keyboard USBs. Mine could just be USB 3.0 though.

3. Power down your Mac and unplug the power (if it is not a MacBook). I have heard this works for some people, though the 2 times I tried it, it did not.

It is unfortunate that this is an issue, and it has cost me time to fix when I could have been working. Once I am able to get my hard drive to connect, I am not able to reconnect it again once my computer goes to sleep. I hope Apple comes out with a patch for this soon. I have ordered a new hard drive and will try to configure it to High Sierra, and hope I no longer have this issue.

I have a LaCie Porsche external hard drive that experienced this same issue. It is partitioned, and the HFS+ partition (with my Time Machine backups and other important content) showed up in Disk Utility as greyed out, refusing to mount on a MBP running High Sierra. I could hear the device spinning, but it seemed stuck.

I typed "sudo lsof|grep disk2" into Terminal, which allowed me to see that the drive was in the middle of a self-initiated "fsck" (file system check) process. I allowed that to run for several hours overnight. By morning, the "fsck"-ing had stopped, and both partitions were available to use.

Hello. I'm not sure if I can help, but here was my situation. For some unknown reason my WD 1TB suddenly stopped "mounting." I'm using MacOS version 10.13.4 (High Sierra). I found one of my older forensic applications called Disk Arbitrator. The application recognized my WD 1TB external hard drive as "unmounted" and I was able to "mount" the drive using this application and I was able to subsequently make a copy of the drive (data) using a software copy program and then "restore" my original drive from the backed-up drive using the "restore" function in Apple's Disk Utility application. The Disk Arbitrator program was an older version (v0.2); I could not figure out this process using "newer" version of this program. BTW, because my drive would not mount on my iMac or MacBook Pro, Apple Tech reached the conclusion that the drive had to be bad and unfortunately I was going to loose all my data. Anyway, this worked for me and I hope it may help someone else. Forgot to mention, Apple Disk Utility was able to see the "unmounted" drive and I was able to perform a "disk repair" check on the drive, but Disk Utility was not able to "mount" the drive - came back with an error message regarding "unable to mount" the drive. Hope this information is helpful.

I went into System Preferences, Spotlight and turned off everything in Search Results. Connected my external HD and it was recognized in finder.

When you are in DU, look in upper left corner an there should be a View option. click on that and you should get an option to Show All Devices. THAT was the key for me as DU then recognized my new external hd and I was able to format and partition it as I wanted.

I cannot believe this is happening AGAIN. Several years ago, I installed an OS update and Time Machine no longer worked with external drives. I contacted Apple via phone and of course, no one knew anything about the problem. After spending hours trying to figure out what was wrong (they tried various steps with me) including taking the machine and drives to the Genius Bar, to no avail, it turned out that the update was the problem and eventually a fix was offered in another update.

So, this time, I waited a LONG time to install High Sierra (today), thinking that the major problems would have been resolved. But I am having the same problems you described, with two drives, although one is recognized but backs up VERY slowly. As I read the replies this problem was introduced with a mid-Dec. update. So even waiting a long time doesn't help! Come on, Apple, this falls well below your standards--and to have it happen twice, is really bad.

As of today this problem still persists. Having done a clean install on a new 2017 MBP, I attached a number of drives to make backups. None of the drives would function normally, except a Drobo which I have used for both Time Machine (TM) Backups and media storage. It has worked almost flawlessly with my 2009 MBP for several years. Having completed my first TM backup yesterday evening, I switched everything on today only for the TM volume not to mount. Disk utility didn't work (first aid, erase or anything else) and I couldn't eject the drive either. When I reattached it to my old MBP I found that the Drobo had been corrupted and have spent several hours trying to get it to work. I found at one point that the TM volume had been reformatted to APFS - apparently as a consequence of setting up a new Time Machine folder (Apple Tech advised me that this journaling is automated - I certainly wasn't given a choice... or a warning). This may be what has caused the issues with the Drobo and is surprising given that TM will not work with APFS!

2 calls to Apple Tech Support failed to resolve the problem and it was clear that they consider problems with 'third party' hardware to be outside their responsibility (so hard luck, apparently). In other words, the MacBook Pro itself is working with High Sierra and problems with hardware outside the Apple eco-system, especially with 'legacy devices', are someone else's problem. I'd pretty much accepted that I needed a new MBP and monitor (+ hub, adapters etc), but certainly didn't expect to have to replace all of my backup drives as well because of an issue with OS compatibility. This seems ridiculous and whilst I like the 2017 MBP in itself, it (and its collection of hangers on) may be going back.

I'm posting this to alert anyone looking to switch across to be mindful that until a fix is confirmed that High Sierra may pose a risk to your data should you attach one or more of your existing drives. It's certainly cost me a lot of time and grief.

Apple... it just works!

I have found the steps given in the link below have resolved issues a few times now when I run into a disk that fails to mount in High Sierra.

https://mycyberuniverse.com/web/how-fix-mediakit-reports-not-enough-space-on-dev ice.html

First time I ran into this issue, it was with a brand new WD Elements HD. I think I eventually found the correct information on Western Digital's website, (which is referenced in the link provided). I ran into this issue again this evening with an old APPLE (Maxtor) hard drive from an old eMac... Seriously Apple - fix your tools... one shouldn't have to find "obscure" terminal commands to do something that DiskUtil should be addressing, particularly if erasing or reformatting a drive... Note that this fix is one to try after other steps (restarting, single user mode disk check, etc.) if the drive has data on it that you need. In my case, I was not concerned with retaining any data from the disks involved.

I have the same problem. I have the very newest MacBook Pro, my WD External hard drives won't mount. In Disk Utilities, the drives are visible, however, "first aid" is unsuccessful and they won't reformat either. In my case, the lights on the external drive flash, and you can hear the internal discs spinning normally.