There are many reasons why TP.exe (the underlying executable that STMS calls on the OS) may hang while trying to make changes to one of the system transport buffers. The reason we came across yesterday was this: there was something corrupt inside the usr/sap/trans/tmp directory. Now, the tmp directory contains a lot of really important stuff for TP.exe, and then again it also can contain a lot of unnecessary junk. I DO NOT recommend deleting this directory. In fact, the best way to fix this is to simply rename that directory and create a new, empty tmp directory, then attempt to manipulate the buffer again with the TP.exe commands or transaction STMS. The tools will work fine with an empty tmp directory, but if you are interested in finding out what file caused this problem you can always copy the files from the original directory one-by-one and run test commands each time to single out which file is the culprit. But I don't have time for any of that. I just left the old tmp directory (now name tmp_old) in place and went about my busy little life.
Hope this helps...
