The 650 main memory was a drum; but what IBM called Random Access Memory (and RAM) for this machine was a hard drive. As described in the Manual of Operation linked above. Here are a few quotes:
"Records in the IBM Random Access Memory Unit are stored on the faces of magnetic disks."
"The stored data in the Random Access Memory Unit are read and written by access arms."
"The IBM 355 RAM units provide extemely large storage capacity for data... Up to four RAM units can be attached to the 650 to provide 24,000,000 digits of RAM storage."
The main memory on the other hand:
"The 20,000 digits of storage, arranges as 2000 words of memory on the magnetic drum..."
"Records in the IBM Random Access Memory Unit are stored on the faces of magnetic disks."
"The stored data in the Random Access Memory Unit are read and written by access arms."
"The IBM 355 RAM units provide extemely large storage capacity for data... Up to four RAM units can be attached to the 650 to provide 24,000,000 digits of RAM storage."
The main memory on the other hand: "The 20,000 digits of storage, arranges as 2000 words of memory on the magnetic drum..."