I am afraid Mepis 7 has a problem with mixed IDE and SATA drives as you will see from the following two links.
This problem has been resolved with Mepis 8 beta (currently 7.9.92-beta5).
A new final version of Mepis 8 is expected in the next 3 or 4 weeks. The M8 beta works perfectly (I have been using it for ages) and you can upgrade to the new version without a fresh install if you wish.
I strongly recommend using the M8 beta (or Mepis 8 final) instead of Mepis 7.
Go to the Mepis download mirror sites: http://www.mepis.org/mirrors
and select your nearest mirror site.
Then select “Testing” followed by a download and burn to CD of: