This post closely follows a previous ASC thread: https://discussions.apple.com/thread/8338922 I have had a very similar problem with a Mac Pro 5,1 (2012), running either Yosemite (10.10.5) or High Sierra (10.13.3) off of SSD or HDD. Upon restart or option-restart, the system will not mount the following disks: 6TB WD Black WD6002FZWZ 8TB HGST He8 HUH728080ALE600 8TB WD Red (white labeled OEM version WD80EMAZ) Other details: - On cold boot or option-boot, all of these drives will mount, and all the drives work great. - The effect is the same with three different systems: Yosemite HFS on SATA 3 sled, High Sierra APFS on PCI card, and Yosemite on a startup volume partition on the WD Red. - None of the hard drives are formatted involving a Logical Volume Group (this was a red herring in the ASC thread). - One of the effected drives (WD Red) has a 2TB partition with Yosemite on it. Upon option-restart, this partition shows up on the option screen and is selectable. If selected, the system gets part way through startup, then a grey circle with line through it appears. This would seem to imply the problem is not a time-out waiting for the drives to spin up as has been suggested in the ASC thread. Rather something is causing the drives to shut down or unmount part way through startup (including the startup partition). - 2 of the 3 drives are SATA 3.3 compliant and have a remote power-off on pin 3. But in all cases pin 3 is manually taped off (and the problem does not occur on cold restart), so I am sure it is not a PWDIS issue. Anyone know what causes this, or have a workaround? It's important that I be able to remotely restart this machine, and not be limited to cold rebooting manually in person each time a restart is required.