After upgrading to OS Sierra I found that my USB3 disks, directly connected to my late 2012 iMac, turn off on startup. USB 2 disks are not affected and connect correctly. The only way to access my USB3 drives is to unplug them and reconnect every time I start the computer then they are ok. I have tried resetting NVRAM but this did not work. I have got back to Os El Capitan until this problem is resolved.
iMac, macOS Sierra (10.12)
Do they not mount or actually turns off while the system is still booting? It wasn't clear from your subject line and subsequent content what was happening.
Are they NTFS formatted drives? See the frequently asked and answered forum solutions if they are.
Thanks for your quick reply.
They are mac formatted.
The USB3 drives actually power down, lights turn off and the disk stops spinning when the apple logo shows up.
USB2 drives are OK.
Under el capitan the USB3 drives are all OK.
I had a similar issue but with an external USB3 startup drive after upgrading it to Sierra. The solution was to connect it to a cheap Chinese USB2 hub just the once, thereafter connecting it directly to the Mac Mini's USB3 port it mounted every time and didn't shut down. Weird.
Thanks for the suggestion, I'll give it a try when I get home tonight.
I am having problems with files on external HDD (not SSDs only HDDs).
Files are there but sometimes are not recognised (by Finder or third party softwares) as what they are. For example a mov file will not open but another one (in the same folder, similar size, same codec etc) will open with no problems. I think the problem is connected to the speed in scanning through the folder. Happens on different disks (that before were working fine). Anyone with similar problem?
Tried plugging into a USB2, hub as suggested, USB3 drives still turn off on start up. I will keep with El Capitan until Apple fix this problem.
I have the same issue. Very frustrating.
External Hard Disk mount issues on MacOS Sierra
