try the MFI recursive flag at the e:/ directory. If there are not too many files on the E drive, it might work to use a file name pattern "FRF.csv"
More than 100K files will probably be slow to hunt them all down.
And yes, you can write a JSL loop. I might use something like this (untested, of course)
foreach( {size}, {"383","533","683"},
foreach({run},{"001","002","003","004","005","006","007","008","009","010"},
name = evalinsert("E:_^size^mm_^run^/FRF.csv") ;
dt = open(name);
...
close(dt,nosave);
)
)
Craige