Hello All, and the fiend. Finally getting to look at this.
iPod no. 1 above when connected to the iMac (via a hub, as it doesn't seem to tolerate the USB C adapter) gave me a split second tantalising glimpse of the iPod's menu screen, i.e. a list of settings on the left half and an image of some sort on the right. Then it went to "Connected, Eject before disconnecting", with the connector symbol in the orange circle.
The battery symbol showed fully charged throughout.
Also a finder window opened, encouragingly with "iPod" in the list of Locations, and a panel stating:-
"Your Mac has detected an iPod which appears to be corrupted. You may need to restore this iPod before it can be used with your Mac etc etc."
Finder stated the software version is 2.0.4 and offered the option to restore the iPod.
Encouraging, but I'd seen this before. I initiated the restore.
It went into recovery mode, the disc drive in the iPod made (irregular) sounds, it alternately showed "Synchronising" and "Connected" (and "Eject before disconnecting" in each case), and the Finder window said
"Software: iPod Recovery Mode
Your iMac is restoring the software on this iPod".
Its shown this for over an hour now, and there's no longer any sound from the iPod's drive.
The previous time I did this process a message came up along the lines of "An unknown error has occurred" with an error code. Not this time, so we'd gone a step further.
I didn't know whether its finished or hung up and next morning the finder window was the same i.e.
"Software: iPod Recovery Mode
Your iMac is restoring the software on this iPod"
and a small pane reading:-
"The disc you attached was not readable by this computer"
with options:-
"Eject', "Ignore" and "Initialise"
Nothing to lose so I chose "Initialise", and got:-
"Your Mac has detected an iPod in recovery mode. You must restore this iPod before it can be used"
Accepting that option, it was back to the page showing version 2.0.4. but this time with a scrolling "barbers pole" at the bottom edge.
Also a window showing "AMP Devices Agent wants to make changes" which I enabled.
I then left the building for a couple of days and now the iPod is responsive. Pressing the Play button I get the screen with the basic menu options at left and on the right, in "About" mode, I see the apple logo and "148 GB free" below.
The menus appear to be fully selectable and the only content is 3 games which I presume were preloaded because I didn't.
So progress has been made.
My music is on an external drive connected to the iMac. The iMac recognised the iPod in Finder. Next I would appreciate steps to transfer the music to the iPod, because the iPod was originally linked to the 2011 iMac.
By way of a reminder, this iPod was number (1) above, which I think I disabled by trying to synch with the 2011 iMac after an OS update which wasn't iPod-compatible.
Thanks again for your patience. I have got this far in resuscitating the iPod thanks to your help.
iPod number (2) is now connected to the iMac, same cable, same port, and after a couple of hours shows full battery charge, "Do Not Disconnect" against a pale background with the flashing circle/diagonal symbol. Pressing centre + MENU, it transitions to the feint apple logo against a grey background and no other button presses do anything, including centre + Left.
Reminder - number (2) just stopped responding after years of successfully playing via an FM transmitter, so the failure mode was different. Its still not recognised in Finder.
Thanks again - Doug