Portable Cygwin: replacing drivename in a text file

[Fergus]

Running Cygwin off a portable USB stick.

Different host machines typically allocate a different drivename to the 
stick.

Previous use, say it was J:. A not-quite-portable application needed a 
settings.ini file built with many occurrences of the string 
j:\usr\local\bin. The file was built and saved.

Current use: now it's K:.

I could re-configure settings.ini with
sed -i 's/j:/k:/g' settings.ini
but in general I don't know (and don't want to bother with) what the 
drivename labels were, or are. Something like
sed -i 's/.:\\/ replacementstring /g' settings.ini
will in fact be sufficiently strong to make the change olddriveletter: 
to newdriveletter: where, and only where, I need it, but experiment as I 
might with
replacementstring = cygpath -w
replacementstring = `cygpath -w`
and other variations, I can't get the syntax quite right.

Can anybody help, please? Thank you very much.

Fergus

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