computer doesn't recognize SSD that replaced optical drive

well, I guess nobody has anything to say on this issue. So I will tell what I have learned: I want to set up a raid0 configuaration but Dell has not given us  E6420 owners  that option. Only a kind of raid1 or a recovery protocol is allowed. I must say I don't like that kind of "tough love". Is there no way to alter the bios so that raid0 is offered? I know the dangers of running raid0. I still want to try it. 

You waited less than 18 hours before concluding that nobody in this community had anything to say?  Give people a bit more time, lol.

Every system I've seen that supports RAID 1 also supports RAID 0, but quite often yes, setting up any RAID level requires you to wipe the involved disks.  Some advanced RAID controllers can set up a RAID 1 by mirroring an existing disk onto a new one without wiping the source disk's data, but that isn't always possible.  However, think carefully about setting up a RAID 0, since that doubles your disk of data loss, because in a RAID 0, if EITHER of those disks has a problem, you lose all of your data.  So at the very least, back up frequently.

But if you don't want to do a RAID, in terms of why that disk isn't showing up in This PC, if it's a new disk, it's because you haven't initialized, partitioned, and formatted it yet.  Go to Disk Management and you should see the disk in the lower part of the main (center) area of the interface.  Right-click the disk "block" at the left of the partition map, select Initialize, and choose MBR or GPT as appropriate (if you don't know which you want, choose MBR if the disk is smaller than 2TB and GPT if it's larger), then you'll be able to create one or more partitions on the disk and format them as desired, presumably as NTFS.  At that point the partition(s) will get drive letter assignments, and then the disk will be visible in This PC.

It worked!  I have added  hard drives (external) before--some of them old and some new--and strange to say, never had a problem. This is the first time something didn't show up. Now I know what to do. I have another of these E6420's that I'm going to alter in the same way but not with the same ignorance and trepidation.  I thank you 1000 times!

Thanks for replying. I will try your suggestion. I was channeled into thinking raid by the notice that kept flashing every time I booted up after installing the new drive.  My end in adding another drive was/is to have more room to store downloads and thus take the pressure off my primary drive--a little guy (small but mighty!)weighing in at just 128 gb. Getting the new drive up and running--a twin to the old--will wake me from my raid nightmare probably forever.

You're quite welcome! :)  External drives often come preformatted for convenience because the expectation is that the customer will just want the entire capacity configured on a single partition.  Internal drives come completely uninitialized because the expectation is there's a reasonable chance an operating system will be installed onto it, and some OS installers work more smoothly and simply if they start with a completely clean slate.  Windows for example creates either 2 or 5 partitions depending on the type of system it's installed on, so if the drive already had a single partition created that spanned the entire drive, the installer would throw an error that Windows couldn't be installed onto that drive, and users would have to know that they had to delete that partition first, which might not be obvious.  Anyhow, glad you're all set!

I was of a generation whose flowering time came when the personal computer was nothing more than a twinkle in God's eye. I was over 40 before I got my first pc-- 40mg hard disk!--back in the early 90's. And even though I have used  every day for maybe 25 years...well, as a friend of mine used to say about playing the guitar, " Start when you're still in diapers...then it's there." Same with computers. But, I am like Goya, "aun aprendo".