Disk utility failing to erase external drive

I have a sneaking suspicion that Time Machine is still trying to use it and that is the volume in the container you can't unmount.

Can you try going into Time Machine's preferences in System Settings while the disk is attached and make sure that you remove "Lyn's Time Machine" as a backup destination? Then see if you can reformat it again.

chuck3400 wrote:

Several attempts to erase/reformat Seagate 4TB external. Wonderful hep from Apple Senior Tech but deadened with referral to Seagate. Seagate Support has morphed into Artificial Stupidity botness and just been frustrating. A possible solution was their "Paragon NTFS" software which disclaimed "Not compatible with apple APFS". Odd?

So what's the solution? The drive is now formatted APFS. Should I proceed and try to erase with Seagate/Paragon?

Advice appreciated!

third party drives often come with proprietary software preinstalled.

It is advised to erase/reformat/initialize the drive as new to get a clean start. You can always download/run NTFS software if you need r/w access NTFS formatted files from Windows after the fact.

If this is mac yes GUID/apfs is the preferred.

DiskUtility verify you are seeing the All Devices. DU>View>Show All Devices you want the top most (parent drive)

Erase and reformat a storage device in Disk Utility on Mac

Erase and reformat a storage device in Disk Utility on Mac - Apple Support

No worries - on your schedule as you feel up to it.

The message about being unable to unmount one of the other volume's in the container is interesting. Could you post a screenshot of what you see in Disk Utility (with the view set to see all devices not just all volumes)? I'd like to see what all the Volumes are on that drive.

Also, if you are trying to erase it, can I assume that you have nothing on the drive you want to keep - should we need to try a blunt approach?

chuck3400 wrote:

Finally managed to erase/reformat (APFS)/eject and remount the drive. Now (IF it's actually reliable) have to repartition for my own use. Based on far better response today from Senate CS I went with a new Seagate 4TB. Are all my trials over? No clue. S**t happens. Sometimes it's major...

You need to read up more about APFS. Traditional partitioning is no longer recommended. Instead just create additional volumes within the APFS container. The advantage over partitioning is that each volume has no limit on storage space. Each volume is allowed to use however much storage it needs up to an including the maximum capacity of the device itself. No more need to increase or decrease partition size to accommodate additional space needs.

Disk Utility User Guide for Mac - Apple Support

You haven't actually said what happens when you try to erase this drive. Is there an error message of some kind? Is the erase button unclickable? Screenshots are always useful.

Glad it worked out in the end.