How can I fix Seagate disk not ejected properly on Mac Pro?

Disk not ejected properly:

This can be caused by a shortcoming of your external Bus Powered drive, and not of your Macintosh computer.

When you Mount any ejectable drive, the Directory from the drive is copied into RAM, and the directory on the drive is marked status = checked out (like a library book that has been borrowed from the lending library). The RAM copy is considered the Master copy, and changes that you make while working are made in the RAM copy of the Directory.

When you Eject/UnMount a drive, the RAM copy of the Directory replaces the copy on the drive, and the status of the directory on the drive is marked status = checked IN and Good.

Some external drives get all their power from the USB Bus. Bus-powered external drives should gracefully transition into standby and accept reduced USB power supplied when your Mac sleeps.

Some drives do this perfectly. Other drives do not transition gracefully and do not run only on standby power. Instead of stand-by, these drives disconnect. The problem is, the Directory from the drive has been checked out, and the Master copy in RAM has no way to get back to the disconnected drive.

Some Time later, when your Mac wakes up again, the drive reports 'I just woke up and am ready to connect'. Your Mac is confused because the drive will not CONTINUE from where it left off, so the Mac says "drive disconnected". The copy of the Directory on the Drive is Stale/checked out (not the Master copy). You get the messages "drive not ejected properly" the same as if you had pulled the cables out while running. The correct state of the Directory can not be determined instantly, but requires Disk Utility 'Repair Disk' procedure.

Summary: this is likely a deficit of the DRIVE, not the Mac. If any one of:

• the drive had external power -OR-

• the drive transitioned gracefully to standby -OR-

• the Mac did not sleep, THEN...

...this would likely not occur.

To resolve, you may need to launch Disk Utility and RepairDisk/FirstAid on the troubled drive.

a similar situation can also occur when a major error occurs on the drive. In that case, the drive is usually made Read-only, until you take steps to Repair it in Disk Utility.

I used to receive the error frequently on the bus-powered WD HDD that I use for Time Machine that is attached to the dock that I use with my MacBook Air. To avoid the issue, I now make sure to eject the drive in Finder (and wait for a few seconds) prior to removing my laptop from the dock.

Thanks Grant Bennet-Alder for such an excellent description of why this error can occur.