Besides the more common EP-ROM cartridge, the was also a development cart using FLASH ROM in the shape of a regular Virtual Boy cartridge. That could be written through the cartridge connector using a Flash Gang Writer.
There were two variants of 8 and 16 MBit, both with 64 KByte of SRAM. That’s 8 times the size of retail carts’ SRAM, which had only 8 KByte.