I have disconnected all my internal hard drives, but left my external USB Hard Drive connected as a solo drive.I then installed Mepis 11 onto the drive followed by Ubuntu 12 as Dual Boot. I then booted my USB drive via the BIOS boot options, and both OS booted and ran without problems.After reconnecting my internal drives and rebooting to my USB drive Ubuntu still loads and runs, but Mepis now hangs on boot with a device not found error.Obviously since reconnecting my drives Mepis is not finding the drive where it expects it to be.It would appear that I need to configure boot/Grub to correct the problem, but I do not know how to do this.Ubuntu obviously modified Grub during installation and created a Boot Loader.When I installed Mepis Grub was written to the MBR.I would be very grateful if someone could help me with this.
My system is an Acer Aspire M3802 Desktop, Intel Q8300 Processor,8GB Ram, Nvidia Geforce GT230 Graphics Card, and four internal SATA Drives (one partitioned into two).On booting the Mepis LiveCD I noted that the USB Drive is enumerated as sdh1, whereas during installation both OS where installed to sdf1.Many Thanks to all.

Could be simply that the ubuntu GRUB didnt find Mepis, or it could be GRUB vs GRUB2.

Running 'update grub' might fix it-

sudo update-grub

Ubuntu has used GRUB2 since 9.10. As far as I know Mepis still uses grub. You can update it to use GRUB2-

http://forum.mepiscommunity.org/view...291534#p291534

Since you installed ubuntu last, that is the grub install controlling your boot.

My bet is that grub is functioning correctly, and booting both OS's fine.

Check the /etc/fstab file of each install.
Ubuntu will designate partitions by UUID, but Mepis is probably using /dev/sdX designation, which will fail if the drive is moved.
Change it to UUID (or better, by LABEL).