O motivo aparece aqui :
Dynamic disks are not supported for USB devices. The help and MSDN say this. You can fake around it in WinXP by attaching a drive to a SATA or IDE port, changing it to dynamic, then putting the drive in a USB enclosure. It appears they decided to enforce this restriction in Win7.
That's a drag, as it will make it difficult to mount a drive from a different machine on a USB chain, which you sometimes need to do to make repairs.
A Microsoft agora está impondo a política não-USB para discos dinâmicos.
Se for um disco dinâmico de disco único (não distribuído ou distribuído para outro disco ou conjunto de discos), você poderá convertê-lo em um disco básico .
To change a dynamic disk back to a basic disk:
- Back up all the data on all the volumes on the disk you want to convert to a basic disk.
- Log on as Administrator or as a member of the Administrators group.
- Click Start, and then click Control Panel.
- Click Performance and Maintenance, click Administrative Tools, and then double-click Computer Management.
- In the left pane, click Disk Management.
- Right-click a volume on the dynamic disk that you want to change to a basic disk, and then click Delete Volume.
- Click Yes when you are prompted to delete the volume.
- Repeat steps 4 and 5 for each volume on the dynamic disk.
- After you have deleted all the volumes on the dynamic disk, right-click the dynamic disk that you want to change to a basic disk, and then click Convert to Basic Disk.
NOTE: You must right-click the gray area that contains the disk title on the left side of the Details pane. For example, right-click Disk 1.