Second hard drive map letter

Question

Since I bought my computer in January the second hard drive where my data is stored has been "D". today I noticed when I opened "my computer" d was assigned to a thumb drive i had inserted and my data appeared to be gone. I shut down and restarted my computer only to find that my data was there but is now named "G". This screws up several programs that look to "D" for data. Please help.