[Epic 4G Q&A, Help & Troubleshooting] bbelos:
Ok, I'll try and clear this up a little bit. ODIN is not technically necessary (I'll update the warning in the CM10 thread).

Background info

CM9 partition sizes: System = 268 MB, Data = 676 MB

CM10 partition sizes: System = 343 MB, Data = 601 MB

The partition sizes had to be modified because CM10 required more room in /system.

Short Story

You can restore a CM9 backup from CM10, because there is enough system space to hold your CM9 system files. WARNING: If the amount of data in your CM9 backup is greater than 601 MB, you will have data loss.

You should not directly restore a CM10 backup from CM9, because there will not be enough system space to hold all the CM10 backup's system files. You WILL have problems.

WORKAROUND: From CM9, you can do an Advanced Boot Restore of a CM10 backup's boot image, then immediately reboot to recovery. Now you have the proper partition sizes, and you can restore a CM10 backup.

Examples scenarios

[FONT="Courier New"]
[B]CM10, current running system.[/B]

Filesystem Size Used Available Use% Mounted on
/dev/block/mtdblock2 [B]343.5M 331.4M[/B] 12.1M 96% /system
/dev/block/mtdblock3 601.5M 467.7M 133.8M 78% /data

*************************************************************************
[B]CM9, wipe everything, clean install.[/B]

Filesystem Size Used Available Use% Mounted on
/dev/block/mtdblock2 [B]268.5M 215.6M[/B] 52.9M 80% /system
/dev/block/mtdblock3 676.5M 59.3M 617.2M 9% /data

**************************************************************************
[B]Restoring CM10 from stock CM9[/B] -> FC's all around from lack of sufficient
system space to restore ([B]trying to jam 331.4M into 268.5M[/B]).

Filesystem Size Used Available Use% Mounted on
/dev/block/mtdblock2 [B]343.5M 268.6M[/B] 74.9M 78% /system
/dev/block/mtdblock3 601.5M 425.6M 175.9M 71% /data

**************************************************************************
[B]Restoring CM10 a second time after stock CM9[/B] -> everything working again.

Filesystem Size Used Available Use% Mounted on
/dev/block/mtdblock2 [B]343.5M 331.4M[/B] 12.1M 96% /system
/dev/block/mtdblock3 601.5M 493.5M 108.0M 82% /data

**************************************************************************
[B]Restoring CM9 from CM10[/B] -> Everything works. [B]Possible data loss[/B] if your
backup contains more than 601.5M of data.

Filesystem Size Used Available Use% Mounted on
/dev/block/mtdblock2 [B] 268.5M 215.6M[/B] 52.9M 80% /system
/dev/block/mtdblock3 676.5M 68.7M 607.8M 10% /data

**************************************************************************
[B]After Advanced CM10 Boot Restore from CM9, then Reboot to
Recovery -> Now there is enough space for a full CM10 restore.[/B]

Filesystem Size Used Available Use% Mounted on
/dev/block/mtdblock2 [B] 343.5M 215.6M[/B] 127.9M 63% /system
/dev/block/mtdblock3 601.5M 68.7M 532.8M 11% /data

**************************************************************************
[B]Restoring CM10 after CM10 Boot restore.[/B]

Filesystem Size Used Available Use% Mounted on
/dev/block/mtdblock2 [B]343.5M 331.4M[/B] 12.1M 96% /system
/dev/block/mtdblock3 601.5M 494.1M 107.4M 82% /data
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