Gilles is spot on when he writes: 

"That used to be a good technique in hte pre-MPE/XL/iX days.

Not necessary any longer.

The O/S will naturally choose whichever disc has the most free space 
for its file disc space allocation."

I poked around in this code as part of the "Large Disk" patch set.   The algorithm is based upon percentage full, that is, we pick the disk in the set with lowest percent full.  Note though this does have a disadvantage that if your volumes are very different sizes as you may bottleneck / serialize lots of I/O to the larger disk(s).   MPE does do better when disks are matched in size.   Also IIRC the original algorithms were designed when 4 GB was a really big disk so were granular to 1/100.    With one of the Large Disk patches I think I made that 1/10000 -- otherwise you'd have to add 3GB of files to a 300GB disk before you'd move to the next volume.  

Jim 

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