A board is not detachable if it has a critical resource (such as a boot drive) connected to it. Similarly, if a system has only one CPU board, the CPU board cannot be detached.
For a device to be detachable:
The device driver must support DDI_DETACH
Critical resources must be accessible through an alternate pathway
If there is no alternate pathway for an I/O board, you can:
Put the second disk chain on a separate I/O board. The secondary I/O board can be detached (with a loss of access to the secondary disk chain).
Add a second path to the device through a second I/O board. The I/O board can be detached (using Alternate Pathing software to switch access through the alternate board) without losing access to the secondary disk chain.