I always have use for the Sandisk SSD on newer Macs so I'll not return it.
OWC's 3G SSDs are actually more expensive than the equal size Sandisks so, if the need was urgent, I'd get one. Certainly the 3G speed would be better but, as the 1.5G speed is quite sufficient in the machine's current use, I'll keep it as is. I know the user will decide to get a new machine in a couple of years so the $200 savings (960GB >> 240GB) will be appreciated when that time comes.
(BTW, 1.5G speed is fine as I regularly upgrade old white MacBooks with SSDs and the improvement is jaw-dropping.)
As for the formatting - Yep, GUID, MacOS Extended, Journaled, non-case-sensitive. I used CCC to clone the El Cap installation from the 240GB Kingston. When that didn't boot in the iMac (and the 240GB Kingston had been booting in other machines just fine), I tried booting from a USB El Cap installer I have used for a number of months but that white screen issue wouldn't even let me bring up the list of bootable drives with the Option key depressed. I removed the Sandisk and installed from scratch on another Mac (with the same USB installer) but returning it to the iMac9,1 resulted in the white screen again. Once I dropped the 240GB Kingston into the iMac, though, the problem was resolved.
It's probably the 3G issue as OWC doesn't even list the imac9,1 on their SSD page; you have to actually select it from a popup menu on that page in order to be brought directly to the 3G SSD page. [Edit: the Kingston is 6G but was probably old enough to have compatibility to older 3G motherboards; newer SSDs don't have this need today.]
I seem to recall a couple of MacBook Pros from the 2009-10 vintage that had this problem, as well. I think Apple issued a firmware update for those units that resolved the issue. I did check for such a firmware update for the iMac but Apple's support area listed the latest update as one that was already installed and an attempted install reported the iMac was already updated.
Thanks for your reply!
Barry