Certain modchips enable the use of a USB hard drive or other mass storage device.
The second, more expensive option, was the HD Combo: it included a replacement case with room for a hard drive. The first was the HD Connect which was simply a board that sat attached to the back of the console, with communication wires taped to the PS2 through its air vents and drive power through one of its 5 V USB headers.
Two special aftermarket kit were created to make this mod easier for the consumer. Third-party connectors can be soldered into the unit giving hard drive support due to still containing some of the necessary I/O controllers and firmware, but the process is involved, requiring the user to solder 39 IDE pinouts directly to the board. Currently only the modified Multitap is sold in stores however, these are also compatible with the older versions, and also added support for multiple memory cards on some games. The official PS2 Linux also requires an expansion bay to function. V7 and V8 included only minor revisions to V6. V5 also introduced a more reliable laser than the ones used in previous models. V5 introduced minor internal changes, and the only difference between V6 (sometimes called V5.1) and V5 is the orientation of the Power/Reset switch board connector, which was reversed to prevent the use of no-solder modchips. In V4, everything except the power supply was unified onto one board. V3 had a substantially different internal structure from the subsequent revisions, featuring several interconnected printed circuit boards.
SCPH-10000 and SCPH-15000 did not have built-in DVD movie playback and instead relied on encrypted playback software that was copied to a memory card from an included CD-ROM (normally, the PS2 will only execute encrypted software from its memory card see PS2 Independence Exploit). These models instead included a PCMCIA slot. Three of the original PS2 launch models (SCPH-10000, SCPH-15000, and SCPH-18000) were only sold in Japan and lacked the expansion bay of later PS2 models.