It appears you’re doing everything exactly right. I tried an equivalent command here, including accessing a remote filesystem mounted in the /Volumes folder and it worked perfectly, creating the file when it didn’t exist.
The most likely reason I can think of for this to fail would be insufficient privileges for accessing or creating the file. You might try some different variations on the command, including creating the file in a folder on the local disk.
Also, be aware that while the open file command will create the file if it doesn’t already exist, the file’s parent folder must already exist. The create file command will create the full path to a file as needed, but in the current version open file won’t. This may change in the future so that all file creation commands will create the full path.
If you need to find out whether the file already exists, you can use the “there is a file” operator, as shown here:
put "/full/path/to/file" into fullPathName
if there is not a file fullPathName then create file fullPathName
open file fullPathName for appending
I hope that helps. Oh, yes, as you noticed, if you don’t specify the full path to a file, it is assumed to be in the current folder (specified by the folder global property).