Hard drive unbootable internally (externally OK)

Hi there !

Apologize for my poor english.

Believe it or not, I've got a problem.

I've a 1TB Western Digital hard drive with OS X installed. When it's pluged in externally with a SATA-USB adapter, it works just fine : I can boot on both OS X and recovery partitions. However when I put the hard drive in my iMac, there is no bootable partition at all... I ran Disk Utility from the OS X installation DVD and here is the output :

Disk :

Disk Description : WDC WD10EZEX-08M2NA0 Media

Bus connection : SATA

Connection type : internal

Capacity : 1 Tb

Write status : read/write

S.M.A.R.T status : checked

Partition map scheme : master boot record (MBR)

A single partition :

Name : DISK0S1

Size : 125 Gb

Format : MS-DOS (FAT)

Status : not mounted

Type : 0xEE

The rest is free space...

When the hard drive is pluged in externaly the output is the following :

Disk Description : WDC WD10EZEX-08M2NA0 Media

Bus connection : USB

Connection type : external

Capacity : 1 Tb

Write status : read/write

S.M.A.R.T status : not managed

Partition map scheme : GUID Partition Table

A single partition :

Name : OSX

Size : 999 Gb

Format : Mac OS extended (journaled)

Status : mounted

Type : -

So the hard drive is detected but what is happening to my OS X partition?