Thanks for the great project, LaserHorse.

My Pi Miner has been running great for the past week with 9 Erupters.   But, I'm having a problem adding more Erupters to it and hope that someone may be able to share some guidance.  

When trying to add 6 additional Erupters (for a total of 15), I discovered that it will only "see" 13 total.  i.e., typing "ls /dev/*USB*" only lists ttyUSB0 - ttyUSB12, even though there are 15 Erupters installed.

A few notes:

• I built a second RPi Miner, with a newly imaged SD-card, and ran into the exact same issue.
• The 15 Erupters were installed across 3 DLink DUB-H7 hubs, 5 in each hub.  So I don't think its power related.
• These 3 hubs feed into a 4th hub which is connected to the RPi.
• I'm using Raspbian, and followed the build process specified in your project walk-through.
• The 2 Erupters that it wont see are running fine on a separate RPi miner.  So it doesn't appear to be a hardware problem with those Erupters.

Any recommendations for troubleshooting this?   Is there a default max number of USB serial devices that are enumerated in Raspbian?