grahame97 Said:
”Not sure what APFS is but as the HD isn't showing in Disk Utility how would I add or reformat or whatever) the disk if the Air can't see it? (Not sure if it is relevant but it is formatted with Journaled but not Encrypted.)“
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You are welcome.
Catalina Reads APFS Format:
APFS is the file system for Catalina. So, using another Catalina-Installed Mac, see if this disk can be seen. If so, then it is probably dust in the port of this computer somewhere. But the other computer - See If you can erase the partitions. Then, format it once more to APFS, in an option.
Too Old of Mac’s for APFS:
If it is too old of a computer, then formatting it to APFS is not an option. So, format it to Extended (Journalized). Then, plugging it into this Mac, format it with APFS. Once APFS is the format type, then Catalina should see it nice and quick. See if it can now be seen on this Mac.
Security Software Getting in the Way:
I don’t know if you have any Security Software installed, but, if so, then it’s Firewall settings are getting in the way here. And such settings would make it just popup and disappear. So, you’d have to uninstall the software. Use the uninstaller and download it form the developer and nowhere else.
Elsewise, your Hard Drive is Failing - it’s time for a new drive.