I want to free up one of the disks I´ve been using for a project. I thought I had copied and moved all relevant material, but when I remove the disk from the system, video files which cant be found anywhere, not on the disk in question, or on any other disk, appear offline in my sequence. Where are these files?

When I try to reconnect the files with the disk unplugged, the location path described starts with "trashes:501(...)". Has anyone had similar problems?

Sounds like you inadvertently moved something necessary to the trash. Open the trash with that disk plugged in, and put the files in a folder on the other drive. Then see if you can reconnect them when you take the external drive offline.

The trash has been emptied with and without the disk plugged in, and the files are still online. The only difference I can detect since my latest emptying of the trashcan, is that the clips on my timeline now displays the "media offline" thumbnail, while it plays as usual, and is still online in my bins.