XPS 8960, 4TB SSD keeps dissapearing!

Do you have the latest BIOS installed?

Do  you have a single 4T SSD or possibly 2x 2T SSDs?  You didn't mention SSD model(s), but is latest firmware update installed?

How can you "restart my computer." and "still not there" ?  If you are able to restart the PC, BIOS must be seeing the boot C: SSD.  Otherwise you wouldn't be able to restart.

Suggest you back up your files or image entire boot SSD ASAP to be safe. 

For starters, try running these commands:

1. At desktop, open a CMD prompt window, Run as administrator
2. At the prompt, type in: chkdsk c: /r and press Enter. Accept offer to run chkdsk at next boot and reboot PC. chkdsk will run before Windows loads, so be patient... If you have more than one drive installed in this PC, repeat chkdsk for each drive by replacing "c:" with the drive letter for each of the other drives, eg d: etc..
3. Back at desktop, open CMD as in #1
4. At the prompt, type in: DISM.exe /Online /Cleanup-image /Restorehealth and press Enter. Be sure to include a space in front of each / and note any error messages when that's done.
5. Assuming no "unfixed" errors in #4, at the CMD prompt, type in: sfc /scannow and press Enter. Be sure to include a space in front of the / and note any errors when that's done.
6. Assuming no "unfixed" errors in #5, reboot and monitor performance.

BTW: Have you scanned thoroughly for malware recently?  Malwarebytes (free) is a good tool for this.

And if that doesn't help and your PC is still under warranty, contact Dell Tech Support. Have your Service Tag available (don't post it here.)

@RoHe​ HI Ron,

Thank you for taking the time out to answer this.  That's very generous. I will look into your directions. 

It is for sure one drive- 4TB.  It seems to be: NVMe TEAM TM8FP4004T. 

It is not the "boot C: SSD".  This is an additional drive that came with it (it came with two drives, the C drive and this 4TB).
By the way, I bought this through "dell store" on amazon.  Amazon refers me to Dell and Dell refuses to assist me.  They tell me that amazon opened up the computer and added the 4TB. I think that is not true as (1) it says it comes from the "dell store" through amazon and (2) it had all the plastic peel covering it.  But even if it is true- it's not my fault- it was advertised as "new" from the dell store.  They don't care.  They refuse to assist.  amazon refuses to assist.  so it's up to me to figure it out.  Anyway, thanks again

That makes more sense.  It's an add-in 4T SSD, not the boot SSD.  AFAIK,  Dell doesn't sell a 4T TEAM NVME SSD, and somebody installed it outside the factory. So Dell isn't responsible. 

What size is the boot SSD?

Run chkdsk c: /r and then chkdsk d: /r (where d: is the drive letter assigned to that 4T SSD). And run the other commands I listed above too. Then test the 4T again. You can also run CrystalDiskInfo (free) after running chkdsk to check that SSD.

Can you find the exact listing for the "Dell Store" where you ordered this PC? I see a lot of offers on Amazon for XPS 8960 but none are actually from Dell. So "Dell Store" may be misleading advertising.  Look on right side under the "Buy Now" button in the offer you bought to see who actually sold the PC. There should be a link to that seller so click it and send them a message through Amazon about the problem.  See if they'll replace that SSD for you.   Also check your Amazon invoice to see if it lists the actual seller's name.

Alternatively, you may need to contact TEAMGROUP. It appears this SSD may have a 5 year warranty...

Dell has a problem with the XPS 8960's.  I'm seeing this with Samsung 990 Pro's (no heatsink) as the boot drive.  Yes the 8960 has the latest BIOS & SSD has the latest firmware.  I've seen this on TWO 8960's and it can take weeks / months for it to happen.  Don't use the excuse "well if Dell doesn't sell it therefore Dell isn't responsible".  I've recommend MANY XPS's, if I have to go another direction then that is what will end up happening.

@btc909​

Dell has a problem with the XPS 8960's.  I'm seeing this with Samsung 990 Pro's (no heatsink) as the boot drive.  Yes the 8960 has the latest BIOS & SSD has the latest firmware.  I've seen this on TWO 8960's and it can take weeks / months for it to happen.  Don't use the excuse "well if Dell doesn't sell it therefore Dell isn't responsible".  I've recommend MANY XPS's, if I have to go another direction then that is what will end up happening.

What problems are you seeing?

Did you reconfigure BIOS and Windows to use AHCI, rather than RAID (factory setting)?  Samsung warns their SSDs and software don't work well when BIOS is set to RAID. Change it this way:

1. Open Cmd prompt window, run as administrator.
2. Copy-paste this command, and Windows starts in Safe Mode next time you boot: bcdedit /set {current} safeboot minimal and press Enter.
3. Restart the computer and tap F2 to enter BIOS setup.
4. Change SATA Operation from RAID to AHCI.
5. Save the change and exit Setup and Windows will automatically boot in  Safe Mode.
6. Open Cmd again, as in step #1.
7. Copy-paste this command, and will start Windows in Normal Mode next time you reboot: bcdedit /deletevalue {current} safeboot and press Enter
8. Reboot and Windows will automatically start with AHCI drivers enabled.

Does that help..?

@RoHe​ I always run AHCI.  I've learned the RAID lesson long ago.

So what problem do you have?