mike the pom wrote:
The WD wouldn't let me copy files onto it, so I took it back to JB HiFi where I bought it. Guy there reformatted it (said it was formatted NTFS).
That's normal for HDDs and SSDs that come reformatted using the Windows-proprietary filesystem NTFS. Out of the box, Macs can read – but not write – volumes using that filesystem. The usual fix is to take a quick trip to Disk Utility to format the drive as APFS (non-case-sensitive) or Mac OS Extended (journaled, non-case-sensitive).
Got home, Finder recognised it but it stopped copying halfway through the first folder.
In the past, Finder hasn't always been the most reliable program to use when copying large numbers of files. I don't know if that has improved in recent versions of macOS, but my preference if I'm copying lots of files and folders in a single batch would be to use Carbon Copy Cloner. (SuperDuper! could probably also do the job.)
Then Finder wouldn't show it on the sidebar.
If you had Finder > {Preferences/Settings} set up to show external drives in the sidebar – and had not ejected that drive – that would be a worrying sign. Might be a "USB drive disconnectted during Sleep" bug – or might be a sign of a drive that is not working correctly and that should not be relied upon for data storage.
The WD is apparently 'Not Mounted', a term above my knowledge level.
Mounted means that a volume is available and ready for use by, applications. Not mounted means that it isn't. As far as most applications are concerned, a volume that is not mounted isn't there – it's the same as if the drive containing the volume wasn't plugged into the computer.
Disk Utility works at a lower level than most applications, so you can use it to mount a volume that is offline – or to unmount a volume (other than the current startup volume) that you want to be offline.
I couldn't make it mount BUT I'm now copying folders in Finder and it appears to be working. The WD is still apparently Not Mounted.
Either you're misinterpreting what you are seeing in Disk Utility - or something is seriously wrong here. The Finder should not showing you unmounted volumes or allowing you to copy folders to, or from them.
It is possible to format a single physical drive such that it contains several volumes, in which case you could have some of them mounted, and some unmounted. But for Disk Utility to say that a volume is Not Mounted while the Finder is letting you use it, is like being told that someone is alive and dead at the same time.
Should I be comfortable that I can delete the source files once they're copied onto the WD?
I absolutely would not be comfortable with deleting the source files.
Something funny is going on here. It sounds like there is a problem with the Finder doing only partial file copies; a problem that might be due to (a) Finder bugs, (b) problems with the drive, or (c) both. There's also this funny stuff with volume mounting status going on.
In its current state, I would not trust the new drive with my data as far as I could throw it.
