Difficulty replacing the hard drive in a Toshiba Satellite A215

I'm helping a relative replace a hard drive in a Satellite A215-S7437. The original drive that came with the computer (Toshiba MK2035GSS) was giving off SMART errors after the recovery disks were also reporting errors during the format/recovery install.

Here are the specs listed on the Toshiba web site for the original drive...

Toshiba MK2035GSS
http://www.sdd.toshiba.com/main.aspx...ives/MK2035GSS

This is the replacement drive I purchased for the machine...

HITACHI Travelstar 7K200 HTS722010K9SA00 (0A50937) 100GB 7200 RPM 16MB Cache 2.5" SATA 1.5Gb/s
http://www.newegg.com/Product/Product.aspx?Item=N82E168...

Here is more information on the laptop specs...

http://www.csd.toshiba.com/cgi-bin/tais/support/jsp/mod...

I removed the old drive from the cage and inserted the new one in the cage and put it in the machine. The laptop is not seeing the new drive at all - either in the bios or on boot up. Then to complicate things, I tried putting the original Toshiba drive back in the machine and it’s now not being recognized either by bios or upon boot.

Would the fact that I bought a drive with less storage than what the computer came with cause it to not be recognized in BIOS (200gb to 100gb)? Also, is it possible I’m putting the drive in incorrectly? It seems to fit snug into the connectors as did the original. Neither drive appears to have jumpers but there are some differences on either side of the connectors.

If anyone can provide any insight as to what needs to be done to get the new drive working properly or if I need to get a different drive it would be most appreciated. If there is something that can be done to fix the original drive, please let me know if that is possible as well.

Thanks in advance for any help that you can provide.