You could have one of two issues. Depending how your external drive is installed (USB, Firewire, SCSI) you may need to re-install those drivers again. Doing so may get the drive to show up.
However, based on what you said "My External drive lists as "Un-allocated" on my computer", I believe that all the "Tech" probalby did since the drive wasn't physically hooked up, was remove the drive from Disk Management. You didn't say how the external drive is connected, but it shouldn't matter. If the system recognizes the external drive, but doesn't give it a drive letter, you can do it manually.
First I want to answer the question you ask "How do I connect my internal drive as a slave?". If your internal drive you are referring to is your "C" drive, you can't make it a slave. I has to be the Master in order for the BIOS to recognize that drive to get to the startup files and Windows. But again, you didn't say how you were connecting your external drive, If you are trying to install the external drive internally (as in pulling it out of an external case and installing in your spare drive bay and hooking it up via you IDE cables), then if doing so doesn't allow it to show up as you claim, then you need to do some reconfiguring in your BIOS so that it wil recognize the drive. If you need to do this, then consult your owners manual, hopefully there will directions there as to how to get into and make changes in your BIOS as well as how to change jumpers on the drive itself to coinside with the setup in the BIOS. If you haven't done this before, be extreemly careful. Making uncertain changes there could cause your PC not to boot. YOu my want to search the net and do some research on this before you try it so as to get some confidence on what you will be doing there.
If the drive is as you say listed as un-allocated, here is what you need to do:
Go to: Control Panel>Administrative Tools>Computer Management. I made a set of windows that shows the steps. Hopefully I did it right that it will show up with this post. If so, it will show you how to get there. In the Computer Management window, on the tree on the left, go down to STORAGE>DISK MANAGEMENT. After you have Disk Management highlighted, you should see a list of your installed drives in the window on the right. You may or may not see the drive in question. Since the "Tech" deleted your drive probably all that will be there is your C drive.
If the drive is there, but doesn't have a letter designation, right click on the drive in the VOLUME column, and select "Change Drive letters and path" (Clicking ACTION>ALL TASKS will get you there also). Then click CHANGE in the next window. The next window the opens should allow you to change (in this case designate) a letter for the drive.
If the drive isn't listed, Click ACTION on the menu on the top (FILE-ACTION-VIEW-WINDOW-HELP), then click on "Rescan Drives" to see if it will recognize the drive. If it does, but doesn't have a drive letter, follow the steps above to give it one.
If running "Rescan Drives" doesn't bring your external drive into the list, right-click on VOLUME, then "Change Drive letters and path", and click ADD (instead of CHANGE). In the next window, click BROWSE. HOPEFULLY, your drive will show in the next list. If it does, select it, then select a drive letter to assign to it. If it doesn't, then you have other problems (the drive probably need to be re-installed or if a USB or Firewire drive, it may need to drivers installed. Go through the earlier steps on installing drivers.
Hope this info helps...Chugbug