Can I put a Samsung 860 EVO SATA M.2 SSD into an XPS 8700 mobo? I see about switching the BIOS from IRST to AHCI for SATA. OTW, will it work? I haven't pulled the side yet to see if there's a slot in the mobo, I assume there is. Has anybody done this?
No - the system doesn't have the M.2 slot that drive needs. You'll need a 2.5" SATA SSD.
While there are also PCIe cards that can adapt M.2 drives, note that if you install one, it's unlikely you will be able to use it as a boot device -- just as storage.
Samsung 860 EVO SATA SSDs come in three physical form factors that I know of:
All of them are SATA III and perform about the same.
The XPS 8700 has:
So, you could get the mSATA format - you might pay more for the same capacity and are limited to 1TB. Or, get the 2.5" format with a mount adaptor. (I've used 2.5" drives connected and running, but loose, in 3.5" bays for a while during testing, etc. until I got a bay adapter to securely mount them.)
Before you do anything, I'll look to see what bays/slots are already used.
Oh, if you go the mSATA route and the slot is empty you'll probably need the screw (and spacer) to hold it down. I don't think Dell supplied the screw if unit was shipped without a mSATA card. Not sure what the screw/spacer part number is.
If you go with a 2.5" SSD you'll probably need a SATA cable.
The question of adding a M.2 or U.2 SSD to the Dell XPS 8700 comes down to this: technically possible, but pointless performance-wise - the XPS 8700 has insufficient PCIe lanes in its PCIe expansion slots.
M.2 and U.2 can utilize 4 PCIe lanes (at PCIe 3.0 speeds). All the PCIe expansion slots in the XPS 8700 are electrically x1 (only hooked up for a single lane), even the one that is physically x4.
In other words, with the XPS 8700, for all the effort to add M.2 or U.2 (I'm talking PCIe NVMe like the Samsung 970 EVO/PRO) you'll get 1/4 of their rated performance, and maybe less if the slots are PCIe 2.0 instead of PCIe 3.0, as I suspect - then we're talking 1/8 of their performance.
A regular 2.5" SATA SSD or mSATA SSD (the Samsung 860 EVO is available in both formats) are your best bets with the XPS 8700.
(The one exception to this is if you're willing to sacrifice the discrete graphics PCIe x16 slot and use it for a M.2 NMVe SSD instead. Your graphics would be limited to the integrated graphics on the CPU.)