The Mojave cannot recognize external hard drive.

Try another USB cable is the easy first try.

I have one drive (single-cable USB 3) that simply WILL NOT mount on a friend's iMac but will on mine reliably. When I swapped cables for a thicker one, that's the only difference I saw, it mounted as expected on both Macs.

You could also try resetting the NVRAM and SMC and also boot into Safe Mode then reboot right after not in Safe Mode.

Safe Mode = Boot with Shift held down until you see "Safe Mode" on screen. This purges lots of cache files. Then reboot again normally.

Safe Mode

https://support.apple.com/en-us/HT201262

NVRAM reset

https://support.apple.com/en-us/HT204063

SMC reset

https://support.apple.com/en-us/HT201295

Sorta often the Mac needs to clear its caches and things after an OS update.

Do you happen to have a Seagate external drive? Seagate is not supporting the latest update to High Sierra either. I found a couple options and I have emailed Seagate. Before I post further let me know if you are using Seagate. We both may be out of luck with new iOS being unsupported.

Zolablue wrote:

Do you happen to have a Seagate external drive? Seagate is not supporting the latest update to High Sierra either. I found a couple options and I have emailed Seagate. Before I post further let me know if you are using Seagate. We both may be out of luck with new iOS being unsupported.

Are you using the Seagate software that came with the drive? If so then you should uninstall it and, if possible, reformat the drive with Disk Utility. The software that comes with Seagate and WD drives are not needed for Macs to use those drives.

That will entail copying the files to some other drive so the drive of interest can be reformatted.

I too, had this issue- perhaps a little different. I updated to Mojave from El Capitan yesterday. For a very long time (at least 2 years) I have been using a 4 port USB hub (not powered/plugged in to AC) connected to my iMac. Connected to this were three different WD external drives. Everything worked perfectly until I switched the OS.

Went through a reinstall of the Mojave OS. Still had a problem with the hub. External drives plugged directly into the iMac via USB worked fine, but not when using the hub. Installed a new multi-port USB hub (powered), and all is now working.

Nobody with Apple Support could identify the problem or explain why the hub suddenly stopped working when switching the OS.

If using multiple external drives, I suggest checking your hub for power, or switching out the hub. Worked for me.

I had the same problem with my powered Anker USB3 hub when updating the OS once. I did an SMC and NVRAM reset and it was recognized again. I'm not sure why but it was like the OS had to be forced to "see" the hub again.

An interesting thing to check when this happens:

Click on Apple Menu (top left)

Hold Option and "About this Mac" will change to "System Information..."

Click on "USB" under "Hardware"

There you can see what the Mac is seeing on your USB chain.

I have Samsung M3 USB3 that The Mojave cannot recognise anymore.

also tried resetting the NVRAM and SMC and also boot into Safe Mode then reboot right after not in Safe Mode, nothing happens. I can see it on USB tree and also in Disk Utility, but can't mount it... in Disk Utility it seems grey and can NOT be mounted.

Any idea?

Hello Zolablue - yes I have a Seagate. Background info: prior to upgrading my Mac to Mojave, my Seagate hard drive could function perfectly fine (on both Mac + PC) and read + edit files (mostly documents, pictures, nothing too big) I had saved from my Windows PC (I work with both laptops) onto my hard drive. Now, my Mac "sees" the Seagate hard drive but it is greyed out and I can not mount it/ not do anything really. I contacted Apple support and they said I had to reformat the whole hard drive. But even if I do so (after saving my docs on a different drive or computer), how will I ever be able to use my saved documents from the hard drive on my Mac? I am lost and need to work on these documents 😟 If anyone can offer advice? Many thanks!!

It is NTFS - used to be read fine - no longer with Mojave. If I format it to OS X Entended - can it work with both Mojave and Windows? (sorry I am really not technical at all so will have to google how to do that if this is the solution you recommend). Many thanks

No, if formatted OS X it can't be used with Windows PC.

It is NTFS

That's why. You had some 3rd party software so you could read and write to it. With the Mojave upgrade that software was removed so it will have to be added again. Make sure to get the latest version

You do know that a Photos or iPhoto library can't be used on a NTFS external hard drive, right. Just a heads up.

Thank you Old Toad. What is the software please? I downloaded iBoySoft but even that can't mount the drive. I am at a loss here as my drive now can only be read by a Windows PC - I am not sure how to get my documents back... Hopefully the software you are talking about is the solution! 🙂 As for the photo library - I don't use iPhoto - just jpg docs. Many thanks for your help!

Paragon Software offers NTFS software for Macs. I've never used it but know many who have poster here do.

The iBoySoft is for exFAT, FAT 32 and FAT formats for PCs. The exFat would be the best selection since it allows for larger file sizes to be managed.

This web site lists a number of free NTFS software: https://iboysoft.com/mac-drive-manager/ntfs-for-mac/free-ntfs-for-mac-os-x.html

I don't know if they are Mojave compatible (maybe RolaringApps.com will know).

Sorry I did not see your reply until now. I had successful backups with my Seagate until the latest High Sierra update a few weeks ago. I need to post the video I found which may help some of you. It did not work for my model of Seagate. I’ll also post my info from Seagate because they admit it no longer works with High Sierra and obviously not with Mojave either.

Had the same issue with my Seagate after updating to 10.14. This site lists all of your options, to include Seagate's free download of Paragon, which saved my life.

https://www.howtogeek.com/236055/how-to-write-to-ntfs-drives-on-a-mac/

Safe mode, NVRAM reset, and SMC reset did not work.

Why did I install Mojave again?

Yes and despite what many write, this is a Mojave OS issue. I can take my external NTFS formatted drive and walk over to wires machine which still runs Sierra, and all works as it always did. Its not the cable, its not the drive format (sort of), etc., its that the latest Apple OS is altered and does not allow the same operations as the previous OS. Now you need a 3rd party app or program to read NTFS formatted drives...so much for touting the "Apples can run Windows etc.". Why Apple, must you always alter new OS in ways that make it less user friendly than previous in one way or another....

No, that is no what the issue is. The issue is Mojave and your answer is about Sierra. My drive worked fine with Sierra, and my drive is a WD, so as has been stated correctly by others, and even by you (Paragon) you need a 3rd party software to mount or access your external drives. This is utter nonsense for a company like Apple that touts being the most advanced etc. No reason for it other than their proprietary tit for tat BS. Of course I want my external drive to work for Windows and Mac both, as the whole world still uses both OS...