I put the new drive in a cheap Kingwin external USB enclosure and formatted it FAT32 with OS X (Disk Utility -> Erase -> MS-DOS Format). (XP will only format FAT32 to 32 GB.) I then used the instructions (in this thread) to install the new drive. It was pretty easy.
I then powered it up and .... It also hung on bootup, until I plugged in the power adapter. Then it spun up and everything seems fine. It's much quieter than the old 20 GB Hitachi drive that came with it.
I'm defragging the old 20 GB drive now. It seems fine.
It's looking like the rechargeable batteries in the Archos are marginal, and the 20 GB drive might have some problems, but it seems OK in an external USB enclosure.
I'll try RockBox when I get some time.
So far, so good...
Cheers,
Scott.