FWIW, I'll pass on some knowledge I gained from one of the Forum Supergurus.
When using Loader 5.0 to restore a remote from the computer, I kept getting "TIU not found on Comm ##" messages, even after using the Loader options menu to search for the TIU, which it found with no problem and promptly identified the comm port ## on which the TIU could be found. I tried 2 TIUs and 2 computers, to no avail.
Superguru advised me that the message can also mean that even if the TIU is found, the Loader program cannot find the remote. A little jiggling of the telephone handset cord in the remote's socket corrected the situation.