What's in your scsi_vhci.conf? We ran into a series of goofy errors with iSCSI and with fibre channel on the same boxes. (Dell SANs). We had to contact the vendor, since Sunacle will not discuss the deep mysteries of the scsi_vhci.conf files with mere customers. Our boxes came up, too, but we saw degradation in I/O. If you keep your I/O stats check 'em before & after.
Anyway, as a wild guess, the syntax you need in the conf file may be slightly different from what you currently have. Even without any change in your storage.
Firstly, what I am about to say below assumes these are standard
"internal" drives and NOT iSCSI or on another RAID controller.
SCSI drives are a huge chunk of electronics and are in themselves highly programmable. All kinds of flags, registers, etc are held on the drive itself in areas technically known as "mode pages". It is possible that some mode page settings got screwed up during your upgrade. This is just a guess.
What you could try is to run the format command, select one of the drives giving a problem, and take the option to "set all mode pages to default".
This won't harm your data.
There was a Qlogic version of a driver in the /kernel/drv/sparcv9 directory which was being loaded on boot, as we were going from update 4 to update 10 on these boxes it could have been any where in between. Removing the offending driver qla2300 resolved the problem, it took sme time to identify the problem but it was finally resolved.
The most likely cause of the problem coming to light is that the kernel had at some point had a bugfix that has highlighted the proble.