Haing a laptop is Dell m5110 , recently i started ot try LMDE, it is indeed a great distro. I have a external usb harddrive (STBV2000300 Seagate® Expansion Desktop 2TB USB 3.0 Drive), this usb drive has always being recognized as 'sda', left the internal hardrive as sdb, although it seems not affects the system booting and runing, but when I access the usb drive, some stuff are missing, and some partions are missing too. so if I want to use this usb harddrive, I have to unplug the usb drive when I booting the system, after booting process then plug it. then it can work properly.
I searched some info, says need to change a bios setting to 'AHCI', I checked it and found it's already at 'AHCI' option.