This one however, I was able to get it working using the AIDv3.01 disc. Booting the X2-5035 BIOS; which probably would've worked with a M8+ as well. Following steps worked like charm

1. recognized/discovered from other threads that the Xenium SUCKS at reading SEAGATE drives for some reason.
2. Solution, don't use Xenium: Drive Upgrade Wizard
3. Leave your original X-HDD plugged in, and backup your E directories and EEPROM that the chip dumps there. (note, i just use the XBox Games Saves Manager to backup T/UDATA DIRs
   note: you can always revert of you don't change your X-HDD
4. Unlock your X-HDD using the Xenium chip
5. Swap the HDDs (remove X-HDD and replace with SEAGATE set to Cable Select)
   note: at this point your new HDD should still be RAW with no partitions
6. Put the AIDv3 disc into the DVD ROM and boot to X2-5035 (obviously you may want to tweak the BIOS to have the F/G partitions set how you like them)
7. Using AIDv3
   > prepare the HDDs (format / prepare)
   > Install the standard MS Dashboard
   > Install whatever alternative DASHs you would like
   > Use the File Explorer in AIDv3 to boot one of your modded DASHs
8. Now, you should see all your HDDs/Partitions as expected/hoped
9. At this point, you will want to create/ftp the X2Config.INI file into the root of F:, and configure how you like/need to.
10. Maybe you want to move some files around, create shortcut XBEs to boot your DASHs, whatever.
11. Make sure that the X2Config is okay to reflect whereever your DASHs are (i suggest shortcuts to XBMC)
12. Remove AIDv3 DVD and reboot
13. Choose the X2-5035 from Xenium and see where it takes you.
14. If it crashes/freezes (give it 1 min just to be sure) just use the AIDv3 DVD again to execute whatever OS you like, and manipulate somethings, maybe try a different BIOS whatever.
15. If you choose to, you can re-lock your HDD too

NOTE 1: The only two BIOSs I would recommend are the M8+ and the X2-5035
NOTE 2: The Xenium CHIP will NEVER see the SEAGATE that I know of. But one the chip as started the BIOS it is no longer part of the picture...so you don't have to worry/about that if you can get AIDv3 to startup for you (...find the "Auto-Installer Deluxe" / "AID" in the usual places)
NOTE 3: um...N-joy.

Your solution is true but statement that Xenium will never see the seagate is not true.

Seems they broke/fixed the seagate thing in XeniumOS 2.0.2 when they did "* Minor changes to IDE/ATAPI handling".

Solution if you want to load bios off your disk without hassle then downgrade your xenium to 2.0.1.

I also noticed a performance increase on writes to the disk after doing this.