guessing you have RAID 1 (mirrors) not RAID 0 (concat or stripe):
you bought the bigger disks usually with the intention to have more space on your filesystems, didn't you? If you just replace disk by disk and use SDS/SVM you just use the same diskspace, even if you offer more space to the metadevice. I'm not sure if SDS changed here, but this was with SDS till Version 4.2.1.
I suggest to run
* fullbackups (my boss will smile when he reads this first point)
* change the system so that it will not boot from metadevices but from hd-slices
* remove the disks from one side of the mirror (usually called the "second" half)
* configure new disks, create filesystems, dump your disks and restore it on the new disks
* boot your new disks, replace small disks by big disks and set up new mirrors. Wait for the resync.
Best Regards,
Franz
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UNIX System Manager from Munich, Germany