Transferring pre-2010 iMac files to 2021 iMac M1 via network connection and external drive

I still have an old pre 2010 iMac which does not have Thunderbolt access (only Firewire 800).

I can't connect my virgin LaCie ext hard drive to it.

I'd like to transfer historical files from this old iMac to my 2021 iMac M1 to then export these to an external 2TB LaCie hard drive connected to the M1.... I can then dump the old Mac.

I have connected boths Mac's via iCloud ... all file share ticked and permitted ... I can see the old Mac files on the M1 but when I try to drag and drop or copy the old Mac files to the virgin LaCie on the M1 the old no entry sign goes up.

The LaCie is apparently pre formatted for Apple Mac products ... the LaCie can well cover the capacity requirement.

Any idiot proof answers please? I'd be very grateful ... thanks

[Re-Titled by Moderator]

Original Title: Old pre 2010 imac files to 2021 iMac M1 via Network connection - unable to drag files to attached 2tb clear LaCie external drive