This pattern is really weird... it's a mixture of checkerboard with vertical lines... what could it be?

Click to expand...

Uninitialised memory.

When a Classic Mac starts up it

(a) first checks the size of memory and does a memory check and

( B) clears the screen using 0xAAAA/0x5555 giving the true 50% chessboard.

© Then the corners get removed as QuickDraw starts it's act, the arrow appears now

(d) then the flashing floppy while looking for a boot up disk,

(e) then a happy mac when it starts the booting.

Yours is right at stage zero.