Hi Russ -- and other contributors.
Very many thanks for your help in nailing this problem. I have fixed it!
Here's what was going on...
I use an external hard disc, "Daily Backup" to back up changed files each morning. After running retrospect, a script in REH ejects the disc (so that it's not read repeatedly, slowing open file dialogs.
But in REH I also use the script "on RetrospectStart."
This checks the list of mounted discs to confirm that Daily Backup is mounted. If it is't, the script uses a shell script command to mount it. The shell script refers to the disc not by name, but by number -- disc1s6.
Now, when I installed Mac OS 10.5.8, I reconnected the data and power cables of an internal disc I hadn't used for some time (so that I could create a bootable duplicate of my 10.4.11 disc.)
This action changed the address of the external disc Daily Backup from disc1s6 to disc2s6.
Now, I shut down my Mac each night, so when I start Retrospect in the morning, Daily Backup is always there. However, if I try to run Retrospect for a second time, Daily Backup is not mounted (because REH ejected it on quit). REH runs, and attempts to mount disc1s6. This was throwing an error which crashed retrospect.
Anyway, I'm very grateful for your help.