7mm hard drive bay

Most drives are currently 9.5mm high. Some new Thinkpads come with 7mm drive bays.

There are many discussions around the internet regarding this, here is a short summary:

Compatible drives

7mm HD

• Hitatch Travelstar series that begin with model code "Z". This includes Z7K320, Z5K500, and Z5K320
• Momentus Thin Hard Drives | Seagate

7mm SSD

• Micron/Crucial C400 series with "AK" in the model number (see below)

       note that Micron is Crucial's parent company. Micron manufacturers the "C" series while Crucial manufactures the "M" series. Reportedly, only the C series is compatible with the x220.

Modifiable 9.5mm SSD

• Micron/Crucial C300 with the spacer mod. See X220 SSD 7mm Mod (Crucial C300)
• Micron/Crucial C400/M4 with spacer mod. (still waiting on confirmation)

       Not all C400 models require a mod. "AK" = 7mm and "AC" = 9mm. Ex) MTFDDAK256MAM-1K1 = 7mm, and MTFDDAC256MAM-1K1 = 9mm. C4 Product Brief
   Intel X-25M with the spacer mod
   Intel 320 series with the spacer mod
   Kingston/Intel X25-E

mSata SSD

• Intel 310 series
• MyDigialSSD (waiting confirmation, relatively unknown brand)
• Renice X3 (SF1222) mSATA SSD User Review
• OCZ Nocti

NOT compatible

• Intel 510 SSD series

• Vertex 2 and 3 SSD series

Case Mod

Some people have also sanded off the protruding parts of the case which stop you inserting a 9.5mm drive without the caddy.

Models Featuring a 7mm Drive Bay

X220