External hard drive not mounting on OS X Sierra

My nice but cheap USB3 enclosure with Toshiba 3.5# HD frequently unmounted, especially when Mac is going to sleep for energy saving. Reset NVRAM did help a litte. But mounting with "Disk Arbitrator" did really help so far.

Yes I did that driver update and it worked Thanks

I have Toshiba HDD. How can I fix this problem?

If it's a Seagate drive, go here and update the driver. http://www.seagate.com/support/external-hard-drives/desktop-hard-drives/backup-p lus-desk/ntfs-driver-for-mac-os-master-dl/

Thank you, Just upgraded to OS Sierra and this fixed my hard drive connectivity.

I have a WD and it as disappeared .... 😟

I've downloaded the Seagate driver but it won't open when I double click it in Finder.

Having similar issues with a Seagate Backup Plus both on a Sierra OS laptop and a Yosemite OS laptop. Installing the latest drivers from Seagate makes no difference.

Interestingly my Toshiba 1TB HD works just fine on both computers.

Tried this but no luck. Followed the visual instructions (no text). Drives verified using there system preferences software. But, no way to check "Disable Access Time" (it's greyed out). The other suggestion is to reformat the drive. What??? How's that a solution? Regardless, this solution doesn't work. Clearly Apple as some 'work' to due themselves to make this "just work".

Posted my issues here before I saw this thread:

macOS Sierra External Drive Mounting Issues

I am having the same issue - i just backed up the majority of my MacBook air - onto a new Toshiba 1TB hard drive - as I was running out of internal memory - then deleted the items - once i had the free space I though great I'll download the new Sierra

Now when i put the hard drive in it doesn't recognise it. tried first aid on it - nothing works 😟 just says not readable

lifesaver -- thank you

so did mine, about three quarters of is full and it shows that it's completely full and i can't mount it

I am having the same issue, but I am using a Toshiba external harddrive.

Model HHDR320E03X